Fair Value Measurements - Fair Value of Other Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair value disclosures
Debt	$ 1,603,192	$ 2,016,139
Current maturities	(20,000)	(20,000)
Long-term debt, excluding current maturities	1,583,192	1,996,139
Carrying amount
Fair value disclosures
Debt	1,603,192	2,016,139
Current maturities	(20,000)	(20,000)
Long-term debt, excluding current maturities	1,583,192	1,996,139
Estimated fair value
Fair value disclosures
Debt	1,506,177	1,839,530
Current maturities	(20,219)	(20,378)
Long-term debt, excluding current maturities	$ 1,485,958	$ 1,819,152