Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets:
Noncurrent derivative instruments	$ 2,517	$ 0
Total assets	156,333	322,337
Liabilities:
Total liabilities	(6,003)	(988)
Recurring
Assets:
Derivative instruments	153,816	322,337
Noncurrent derivative instruments	2,517
Total assets	156,333	322,337
Liabilities:
Derivative instruments	(3,079)	(988)
Noncurrent derivative instruments	(2,924)
Total liabilities	(6,003)	(988)
Net derivative asset (liability)	150,330	321,349
Recurring | Level 2
Assets:
Derivative instruments	164,475	294,865
Noncurrent derivative instruments	3,487
Total assets	167,962	294,865
Liabilities:
Derivative instruments	0	2,048
Noncurrent derivative instruments	0
Total liabilities	0	2,048
Net derivative asset (liability)	167,962	296,913
Recurring | Level 3
Assets:
Derivative instruments	(10,659)	27,472
Noncurrent derivative instruments	(970)
Total assets	(11,629)	27,472
Liabilities:
Derivative instruments	(3,079)	(3,036)
Noncurrent derivative instruments	(2,924)
Total liabilities	(6,003)	(3,036)
Net derivative asset (liability)	$ (17,632)	$ 24,436