Debt and Letters of Credit - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current:
Other borrowings	$ 4,639	$ 4,890
Long-term:
Borrowings under credit facility	0	0
Long-term debt	1,170,544	1,701,098
Total long-term	1,170,544	1,701,098
2023 Notes
Long-term:
Long-term debt	196,589	611,250
Unamortized discount	(61)	(283)
Unamortized deferred financing costs	(257)	(1,203)
2024 Notes
Long-term:
Long-term debt	381,014	500,000
Unamortized discount	(1,316)	(2,130)
Unamortized deferred financing costs	(1,032)	(1,670)
2028 Notes
Long-term:
Long-term debt	600,000	600,000
Unamortized discount	(886)	(981)
Unamortized deferred financing costs	$ (3,507)	$ (3,885)