Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivative Asset [Abstract]
Total assets	$ 147,789	$ 322,337
Derivative Liability [Abstract]
Total liabilities	(14,105)	(988)
Recurring
Derivative Asset [Abstract]
Derivative instruments	147,789	322,337
Total assets	147,789	322,337
Derivative Liability [Abstract]
Derivative instruments	(10,220)	(988)
Noncurrent derivative instruments	(3,885)
Total liabilities	(14,105)	(988)
Net derivative asset	133,684	321,349
Recurring | Level 2
Derivative Asset [Abstract]
Derivative instruments	145,979	294,865
Total assets	145,979	294,865
Derivative Liability [Abstract]
Derivative instruments	1,069	2,048
Noncurrent derivative instruments	699
Total liabilities	1,768	2,048
Net derivative asset	147,747	296,913
Recurring | Level 3
Derivative Asset [Abstract]
Derivative instruments	1,810	27,472
Total assets	1,810	27,472
Derivative Liability [Abstract]
Derivative instruments	(11,289)	(3,036)
Noncurrent derivative instruments	(4,584)
Total liabilities	(15,873)	(3,036)
Net derivative asset	$ (14,063)	$ 24,436