Derivative Financial Instruments - Impact of Derivative Financial Instruments within Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Net settlements on matured derivatives	[1]	$ 357	$ 6,191	$ 201	$ 17,285
Net change in fair value of derivatives		2,176	(16,279)	5,464	(24,805)
Total gain (loss) on derivatives, net		$ 2,533	$ (10,088)	$ 5,665	$ (7,520)

[1] The settlement amount does not include premiums paid attributable to contracts that matured during the respective period.