Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Derivative instruments	$ 35,377	$ 0
Noncurrent derivative instruments	258	0
Net Amounts Presented in the Balance Sheets	35,635	0
Liabilities:
Net Amount Presented in the Balance Sheets	112,031	80,265
Recurring
Assets:
Derivative instruments	35,377
Noncurrent derivative instruments	258
Net Amounts Presented in the Balance Sheets	35,635
Liabilities:
Derivative instruments	80,996	71,379
Noncurrent derivative instruments	31,035	8,886
Net Amount Presented in the Balance Sheets	112,031	80,265
Net derivative asset (liability)	(76,396)	(80,265)
Recurring | Level 2
Assets:
Derivative instruments	(40,851)
Noncurrent derivative instruments	(2,037)
Net Amounts Presented in the Balance Sheets	(42,888)
Liabilities:
Derivative instruments	95,321	43,241
Noncurrent derivative instruments	28,490	7,736
Net Amount Presented in the Balance Sheets	123,811	50,977
Net derivative asset (liability)	(166,699)	(50,977)
Recurring | Level 3
Assets:
Derivative instruments	76,228
Noncurrent derivative instruments	2,295
Net Amounts Presented in the Balance Sheets	78,523
Liabilities:
Derivative instruments	(14,325)	28,138
Noncurrent derivative instruments	2,545	1,150
Net Amount Presented in the Balance Sheets	(11,780)	29,288
Net derivative asset (liability)	$ 90,303	$ (29,288)