Commodity Derivative Financial Instruments - Changes in Fair Value of Company's Commodity Derivative Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives not designated as hedging instruments
Gain (loss) on derivative instruments	$ 26,902	$ (36,464)	$ 90,288
Net cash (received) paid on settlements of derivative instruments	(15,211)	(44,789)	(63,225)
Not Designated as Hedging Instrument
Derivatives not designated as hedging instruments
Beginning fair value of commodity derivative instruments	(16,719)	64,534	37,471
Net change in fair value of commodity derivative instruments	11,691	(81,253)	27,063
Ending fair value of commodity derivative instruments	(5,028)	(16,719)	64,534
Oil | Not Designated as Hedging Instrument
Derivatives not designated as hedging instruments
Gain (loss) on derivative instruments	(5,091)	(15,998)	57,681
Net cash (received) paid on settlements of derivative instruments	(10,901)	(27,450)	(41,786)
Natural Gas | Not Designated as Hedging Instrument
Derivatives not designated as hedging instruments
Gain (loss) on derivative instruments	31,993	(20,466)	32,607
Net cash (received) paid on settlements of derivative instruments	$ (4,310)	$ (17,339)	$ (21,439)