Fair Value Measurements - Debt Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair value disclosures
Long-term debt	$ 1,520,530	$ 2,016,139
Current maturities	0	(20,000)
Long-term debt, net	1,520,530	1,996,139
Carrying Amount
Fair value disclosures
Long-term debt	1,520,530	2,016,139
Current maturities	0	(20,000)
Long-term debt, net	1,520,530	1,996,139
Estimated Fair Value
Fair value disclosures
Long-term debt	1,463,643	1,839,530
Current maturities	0	(20,378)
Long-term debt, net	$ 1,463,643	$ 1,819,152