Fair Value Measurements - Debt Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair value disclosures
Long-term debt	$ 1,521,891	$ 1,520,530
Current maturities	(304,000)	0
Long-term debt, net	1,217,891	1,520,530
Carrying Amount
Fair value disclosures
Long-term debt	1,521,891	1,520,530
Current maturities	(304,000)	0
Long-term debt, net	1,217,891	1,520,530
Estimated Fair Value
Fair value disclosures
Long-term debt	1,527,624	1,463,643
Current maturities	(312,055)	0
Long-term debt, net	$ 1,215,569	$ 1,463,643