Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Noncurrent derivative instruments	$ 3,261	$ 0
Net Amounts Presented in the Balance Sheets	8,245	0
Liabilities:
Net Amount Presented in the Balance Sheets	69,041	80,265
Recurring
Assets:
Derivative instruments	4,984
Noncurrent derivative instruments	3,261
Net Amounts Presented in the Balance Sheets	8,245
Liabilities:
Derivative instruments	57,478	71,379
Noncurrent derivative instruments	11,563	8,886
Net Amount Presented in the Balance Sheets	69,041	80,265
Net derivative asset/liability	(60,796)	(80,265)
Recurring | Level 2
Assets:
Derivative instruments	0
Noncurrent derivative instruments	(2,283)
Net Amounts Presented in the Balance Sheets	(2,283)
Liabilities:
Derivative instruments	68,782	43,241
Noncurrent derivative instruments	14,233	7,736
Net Amount Presented in the Balance Sheets	83,015	50,977
Net derivative asset/liability	(85,298)	(50,977)
Recurring | Level 3
Assets:
Derivative instruments	4,984
Noncurrent derivative instruments	5,544
Net Amounts Presented in the Balance Sheets	10,528
Liabilities:
Derivative instruments	(11,304)	28,138
Noncurrent derivative instruments	(2,670)	1,150
Net Amount Presented in the Balance Sheets	(13,974)	29,288
Net derivative asset/liability	$ 24,502	$ (29,288)