Fair Value Measurements - Fair Value of Other Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair value disclosures
Long-term debt	$ 1,522,231	$ 1,521,891
Current maturities	(304,000)	(304,000)
Long-term debt, excluding current maturities	1,218,231	1,217,891
Carrying Amount
Fair value disclosures
Long-term debt	1,522,231	1,521,891
Current maturities	(304,000)	(304,000)
Long-term debt, excluding current maturities	1,218,231	1,217,891
Estimated Fair Value
Fair value disclosures
Long-term debt	1,499,909	1,527,624
Current maturities	(308,846)	(312,055)
Long-term debt, excluding current maturities	$ 1,191,063	$ 1,215,569