Fair Value Measurements - Fair Value of Other Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair value disclosures
Debt, net	$ 1,539,849	$ 2,016,139
Current maturities	(20,000)	(20,000)
Long-term debt, excluding current maturities	1,519,849	1,996,139
Carrying amount
Fair value disclosures
Debt, net	1,539,849	2,016,139
Current maturities	(20,000)	(20,000)
Long-term debt, excluding current maturities	1,519,849	1,996,139
Estimated fair value
Fair value disclosures
Debt, net	1,519,226	1,839,530
Current maturities	(20,060)	(20,378)
Long-term debt, excluding current maturities	$ 1,499,166	$ 1,819,152