| Summary of long-term debt | 
Long-term debt, which includes capital leases, at September 30, 2018 and December 31, 2017, consists of the following:
 
|  |  
|  |  |  |  |  |  |  |  |  
|   | September 30, 2018 |   | December 31, 2017 |  
|   | (in thousands) |  
| Senior Secured Credit Facility: |   |   |   |  
|   Term loan A | $ | 161,500 | 
 |   | $ | 175,750 | 
 |  
|   Term loan B | 955,573 | 
 |   | 962,849 | 
 |  
| 4.875% Senior Notes due 2024 | 575,000 | 
 |   | 575,000 | 
 |  
| 5.625% Senior Notes due 2026 | 300,000 | 
 |   | — | 
 |  
| 5.375% Senior Notes due 2022 | 250,000 | 
 |   | 250,000 | 
 |  
| 2.5% Convertible Senior Notes due 2023 | 550,000 | 
 |   | — | 
 |  
| 2.5% Convertible Senior Notes due 2019 | 28,674 | 
 |   | 275,000 | 
 |  
| Other long-term debt | 93,398 | 
 |   | 99,393 | 
 |  
| Total principal amount | 2,914,145 | 
 |   | 2,337,992 | 
 |  
| Less unamortized discounts and debt issuance costs | (100,328 | ) |   | (38,033 | ) |  
| Total debt, net of unamortized discounts and debt issuance costs | 2,813,817 | 
 |   | 2,299,959 | 
 |  
| Less: current portion | 81,832 | 
 |   | 347,593 | 
 |  
| Total long-term debt, net of unamortized discounts and debt issuance costs | $ | 2,731,985 | 
 |   | $ | 1,952,366 | 
 |  | 
| Future maturities of long-term debt | 
Future maturities of long-term debt at September 30, 2018 are as follows:
 
|  |  
|  |  |  |  |  
|   | (in thousands) |  
| October 1 - December 31, 2018 | $ | 51,802 | 
 |  
| 2019 | 45,192 | 
 |  
| 2020 | 71,091 | 
 |  
| 2021 | 122,671 | 
 |  
| 2022 | 819,010 | 
 |  
| Thereafter | 1,804,379 | 
 |  
| Total | $ | 2,914,145 | 
 |  | 
| Summary of pre tax interest cost recognized on convertible senior notes | 
The following table summarizes the amount of pre-tax interest cost recognized on the notes: 
|  |  
|  |  |  |  |  |  |  |  |  
|   | Three Months Ended September 30, 2018
 |   | Nine Months Ended September 30, 2018
 |  
|   | (in thousands) |  
| Interest cost recognized relating to: |   |   |   |  
|   Contractual interest coupon | $ | 3,361 | 
 |   | $ | 7,257 | 
 |  
|   Amortization of debt discount | 2,996 | 
 |   | 5,951 | 
 |  
|   Amortization of debt issuance costs | 534 | 
 |   | 1,068 | 
 |  
| Total interest cost recognized on the notes | $ | 6,891 | 
 |   | $ | 14,276 | 
 |  |