| CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY  (UNAUDITED) - USD ($)$ in Thousands
 | Total | Common Stock [Member] | Additional Paid-in Capital [Member] | Accumulated Deficit [Member] | Cost of Shares Held in Treasury [Member] | Accumulated Other Comprehensive Loss [Member] | Noncontrolling Interests [Member] | Redeemable Noncontrolling Interests [Member] | 
| Balances at Dec. 31, 2017 | $ 1,418,144 | $ 2,069 | $ 2,374,006 | $ (1,079,472) | $ (6,865) | $ (108,542) | $ 236,948 |  | 
| Balances (in shares) at Dec. 31, 2017 |  | 206,877,037 |  |  |  |  |  |  | 
| Equity [Roll Forward] |  |  |  |  |  |  |  |  | 
| Non-cash and stock-based compensation | 23,320 |  | 23,320 |  |  |  |  |  | 
| Common Stock issued under stock plans, net of shares withheld for employee taxes | (8,602) | $ 4 | (8,606) |  |  |  |  |  | 
| Common stock issued under stock plans, net of shares withheld for employee taxes (in shares) |  | 359,817 |  |  |  |  |  |  | 
| Exercise of stock options, net of shares withheld for option cost and employee taxes | 10,854 | $ 6 | 10,848 |  |  |  |  |  | 
| Exercise of stock options (in shares) |  | 632,981 |  |  |  |  |  |  | 
| Fair value of convertible debt conversion feature, net of issuance costs | 62,624 |  | 62,624 |  |  |  |  |  | 
| Repurchase of convertible debt conversion feature | (92,641) |  | (92,641) |  |  |  |  |  | 
| Acquisitions | 22,404 |  |  |  |  |  | 22,404 |  | 
| Purchases of noncontrolling interests | 73 |  | 1,709 |  |  |  | (1,636) |  | 
| Redeemable noncontrolling interests fair value adjustments | (33,456) |  | (33,456) |  |  |  |  |  | 
| Contributions received | 4,900 |  |  |  |  |  | 4,900 |  | 
| Cash distributions | (23,269) |  |  |  |  |  | (23,269) |  | 
| Other | (2,571) |  | (113) |  |  |  | (2,458) |  | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income | 41,035 |  |  | 35,453 |  |  | 5,582 |  | 
| Foreign currency translation adjustments | (17,295) |  |  |  |  | (17,295) |  |  | 
| Balances at Jun. 30, 2018 | 1,405,520 | $ 2,079 | 2,337,691 | (1,044,019) | (6,865) | (125,837) | 242,471 |  | 
| Balances (in shares) at Jun. 30, 2018 |  | 207,869,835 |  |  |  |  |  |  | 
| Balances at Dec. 31, 2017 |  |  |  |  |  |  |  | $ 244,727 | 
| Redeemable Noncontrolling Interests [Roll Forward] |  |  |  |  |  |  |  |  | 
| Acquisitions |  |  |  |  |  |  |  | 21,870 | 
| Purchases of noncontrolling interests |  |  |  |  |  |  |  | (356) | 
| Redeemable noncontrolling interests fair value adjustments |  |  |  |  |  |  |  | 33,456 | 
| Contributions received |  |  |  |  |  |  |  | 0 | 
| Cash distributions |  |  |  |  |  |  |  | (7,874) | 
| Other |  |  |  |  |  |  |  | 78 | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income |  |  |  |  |  |  |  | 1,293 | 
| Balances at Jun. 30, 2018 |  |  |  |  |  |  |  | 293,194 | 
| Balances at Mar. 31, 2018 | 1,372,221 | $ 2,076 | 2,348,118 | (1,113,378) | (6,865) | (92,280) | 234,550 |  | 
| Balances (in shares) at Mar. 31, 2018 |  | 207,634,062 |  |  |  |  |  |  | 
| Equity [Roll Forward] |  |  |  |  |  |  |  |  | 
| Non-cash and stock-based compensation | 12,450 |  | 12,450 |  |  |  |  |  | 
| Common Stock issued under stock plans, net of shares withheld for employee taxes | (656) | $ 1 | (657) |  |  |  |  |  | 
| Common stock issued under stock plans, net of shares withheld for employee taxes (in shares) |  | 72,008 |  |  |  |  |  |  | 
| Exercise of stock options, net of shares withheld for option cost and employee taxes | 2,932 | $ 2 | 2,930 |  |  |  |  |  | 
| Exercise of stock options (in shares) |  | 163,765 |  |  |  |  |  |  | 
| Repurchase of convertible debt conversion feature | (9,225) |  | (9,225) |  |  |  |  |  | 
| Acquisitions | 8,955 |  |  |  |  |  | 8,955 |  | 
| Purchases of noncontrolling interests | 0 |  | 1,143 |  |  |  | (1,143) |  | 
| Redeemable noncontrolling interests fair value adjustments | (16,924) |  | (16,924) |  |  |  | 0 |  | 
| Cash distributions | (6,992) |  |  |  |  |  | (6,992) |  | 
| Other | (2,611) |  | (144) |  |  |  | (2,467) |  | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income | 78,927 |  |  | 69,359 |  |  | 9,568 |  | 
| Foreign currency translation adjustments | (33,557) |  |  |  |  | (33,557) |  |  | 
| Balances at Jun. 30, 2018 | 1,405,520 | $ 2,079 | 2,337,691 | (1,044,019) | (6,865) | (125,837) | 242,471 |  | 
| Balances (in shares) at Jun. 30, 2018 |  | 207,869,835 |  |  |  |  |  |  | 
| Balances at Mar. 31, 2018 |  |  |  |  |  |  |  | 264,700 | 
| Redeemable Noncontrolling Interests [Roll Forward] |  |  |  |  |  |  |  |  | 
| Acquisitions |  |  |  |  |  |  |  | 11,689 | 
| Purchases of noncontrolling interests |  |  |  |  |  |  |  | 2 | 
| Redeemable noncontrolling interests fair value adjustments |  |  |  |  |  |  |  | 16,924 | 
| Cash distributions |  |  |  |  |  |  |  | (4,628) | 
| Other |  |  |  |  |  |  |  | 78 | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income |  |  |  |  |  |  |  | 4,429 | 
| Balances at Jun. 30, 2018 |  |  |  |  |  |  |  | 293,194 | 
| Balances at Dec. 31, 2018 | 1,342,743 | $ 2,091 | 2,268,209 | (1,019,223) | (6,865) | (145,231) | 243,762 |  | 
| Balances (in shares) at Dec. 31, 2018 |  | 209,135,581 |  |  |  |  |  |  | 
| Equity [Roll Forward] |  |  |  |  |  |  |  |  | 
| Non-cash and stock-based compensation | 24,992 |  | 24,992 |  |  |  |  |  | 
| Common Stock issued under stock plans, net of shares withheld for employee taxes | (10,259) | $ 3 | (10,262) |  |  |  |  |  | 
| Common stock issued under stock plans, net of shares withheld for employee taxes (in shares) |  | 312,857 |  |  |  |  |  |  | 
| Exercise of stock options, net of shares withheld for option cost and employee taxes | 10,199 | $ 7 | 10,192 |  |  |  |  |  | 
| Exercise of stock options (in shares) |  | 637,377 |  |  |  |  |  |  | 
| Conversion of convertible debt | 28,586 | $ 0 | 28,586 |  |  |  |  |  | 
| Conversion of convertible debt (in shares) |  | 824,328 |  |  |  |  |  |  | 
| Acquisitions | 26,168 |  |  |  |  |  | 26,168 |  | 
| Purchases of noncontrolling interests | 0 |  | 270 |  |  |  | (270) |  | 
| Redeemable noncontrolling interests fair value adjustments | (25,827) |  | (25,827) |  |  |  |  |  | 
| Contributions received | 8,324 |  |  |  |  |  | 8,324 |  | 
| Cash distributions | (52,115) |  |  |  |  |  | (52,115) |  | 
| Other | (1,411) |  | (150) |  |  |  | (1,261) |  | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income | 59,916 |  |  | 50,911 |  |  | 9,005 |  | 
| Foreign currency translation adjustments | (3,560) |  |  |  |  | (3,560) |  |  | 
| Balances at Jun. 30, 2019 | 1,407,756 | $ 2,101 | 2,296,010 | (968,312) | (6,865) | (148,791) | 233,613 |  | 
| Balances (in shares) at Jun. 30, 2019 |  | 210,910,143 |  |  |  |  |  |  | 
| Balances at Dec. 31, 2018 | 329,355 |  |  |  |  |  |  | 329,355 | 
| Redeemable Noncontrolling Interests [Roll Forward] |  |  |  |  |  |  |  |  | 
| Acquisitions |  |  |  |  |  |  |  | 39,303 | 
| Purchases of noncontrolling interests |  |  |  |  |  |  |  | (1,459) | 
| Redeemable noncontrolling interests fair value adjustments |  |  |  |  |  |  |  | 25,827 | 
| Contributions received |  |  |  |  |  |  |  | 0 | 
| Cash distributions |  |  |  |  |  |  |  | (11,258) | 
| Other |  |  |  |  |  |  |  | 2,597 | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income |  |  |  |  |  |  |  | 963 | 
| Balances at Jun. 30, 2019 | 385,328 |  |  |  |  |  |  | 385,328 | 
| Balances at Mar. 31, 2019 | 1,240,394 | $ 2,099 | 2,267,761 | (1,071,667) | (6,865) | (148,120) | 197,186 |  | 
| Balances (in shares) at Mar. 31, 2019 |  | 209,857,816 |  |  |  |  |  |  | 
| Equity [Roll Forward] |  |  |  |  |  |  |  |  | 
| Non-cash and stock-based compensation | 11,754 |  | 11,754 |  |  |  |  |  | 
| Common Stock issued under stock plans, net of shares withheld for employee taxes | (959) | $ 0 | (959) |  |  |  |  |  | 
| Common stock issued under stock plans, net of shares withheld for employee taxes (in shares) |  | 60,223 |  |  |  |  |  |  | 
| Exercise of stock options, net of shares withheld for option cost and employee taxes | 2,527 | $ 2 | 2,525 |  |  |  |  |  | 
| Exercise of stock options (in shares) |  | 167,776 |  |  |  |  |  |  | 
| Conversion of convertible debt | 28,586 | $ 0 | 28,586 |  |  |  |  |  | 
| Conversion of convertible debt (in shares) |  | 824,328 |  |  |  |  |  |  | 
| Acquisitions | 25,194 |  |  |  |  |  | 25,194 |  | 
| Purchases of noncontrolling interests | 0 |  | 0 |  |  |  | 0 |  | 
| Redeemable noncontrolling interests fair value adjustments | (13,515) |  | (13,515) |  |  |  |  |  | 
| Contributions received | 8,324 |  |  |  |  |  | 8,324 |  | 
| Cash distributions | (6,172) |  |  |  |  |  | (6,172) |  | 
| Other | (219) |  | (142) |  |  |  | (77) |  | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income | 112,513 |  |  | 103,355 |  |  | 9,158 |  | 
| Foreign currency translation adjustments | (671) |  |  |  |  | (671) |  |  | 
| Balances at Jun. 30, 2019 | 1,407,756 | $ 2,101 | $ 2,296,010 | $ (968,312) | $ (6,865) | $ (148,791) | $ 233,613 |  | 
| Balances (in shares) at Jun. 30, 2019 |  | 210,910,143 |  |  |  |  |  |  | 
| Balances at Mar. 31, 2019 |  |  |  |  |  |  |  | 343,029 | 
| Redeemable Noncontrolling Interests [Roll Forward] |  |  |  |  |  |  |  |  | 
| Acquisitions |  |  |  |  |  |  |  | 32,322 | 
| Purchases of noncontrolling interests |  |  |  |  |  |  |  | 0 | 
| Redeemable noncontrolling interests fair value adjustments |  |  |  |  |  |  |  | 13,515 | 
| Contributions received |  |  |  |  |  |  |  | 0 | 
| Cash distributions |  |  |  |  |  |  |  | (7,317) | 
| Other |  |  |  |  |  |  |  | 172 | 
| Comprehensive income (loss): |  |  |  |  |  |  |  |  | 
| Net income |  |  |  |  |  |  |  | 3,607 | 
| Balances at Jun. 30, 2019 | $ 385,328 |  |  |  |  |  |  | $ 385,328 |