Derivative Instruments - Impact of Netting Agreements and Margin Deposits (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Asset derivatives:
Derivative instruments, at fair value	$ 349,152	$ 458,460
Liability derivatives:
Derivative instruments, at fair value	43,696	22,942
Commodity Derivatives
Asset derivatives:
Derivative instruments, at fair value	349,152	458,460
Derivative instruments subject to master netting agreements	(27,724)	(22,810)
Margin deposits remitted to counterparties	0	0
Derivative instruments, net	321,428	435,650
Liability derivatives:
Derivative instruments, at fair value	43,696	22,942
Derivative instruments subject to master netting agreements	(27,724)	(22,810)
Margin deposits remitted to counterparties	0	(132)
Derivative instruments, net	$ 15,972	$ 0