Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Derivative instruments	$ 4,226	$ 0
Net Amounts Presented in the Balance Sheets	4,226	0
Liabilities:
Net Amount Presented in the Balance Sheets	230,229	80,265
Recurring
Assets:
Derivative instruments	4,226
Net Amounts Presented in the Balance Sheets	4,226
Liabilities:
Derivative instruments	172,772	71,379
Noncurrent derivative instruments	57,457	8,886
Net Amount Presented in the Balance Sheets	230,229	80,265
Net derivative liability	(226,003)	(80,265)
Recurring | Level 2
Assets:
Derivative instruments	(4,645)
Net Amounts Presented in the Balance Sheets	(4,645)
Liabilities:
Derivative instruments	167,065	43,241
Noncurrent derivative instruments	30,787	7,736
Net Amount Presented in the Balance Sheets	197,852	50,977
Net derivative liability	(202,497)	(50,977)
Recurring | Level 3
Assets:
Derivative instruments	8,871
Net Amounts Presented in the Balance Sheets	8,871
Liabilities:
Derivative instruments	5,707	28,138
Noncurrent derivative instruments	26,670	1,150
Net Amount Presented in the Balance Sheets	32,377	29,288
Net derivative liability	$ (23,506)	$ (29,288)