Derivative Instruments - Cash and Non-Cash Gains and Losses on Derivative Instruments (Detail) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Fixed price swaps Crude Oil [Member]	Sep. 30, 2013 Fixed price swaps Crude Oil [Member]	Sep. 30, 2014 Fixed price swaps Crude Oil [Member]	Sep. 30, 2013 Fixed price swaps Crude Oil [Member]	Sep. 30, 2014 Fixed price swaps Natural Gas [Member]	Sep. 30, 2013 Fixed price swaps Natural Gas [Member]	Sep. 30, 2014 Fixed price swaps Natural Gas [Member]	Sep. 30, 2013 Fixed price swaps Natural Gas [Member]	Sep. 30, 2014 Collars Crude Oil [Member]	Sep. 30, 2013 Collars Crude Oil [Member]	Sep. 30, 2014 Collars Crude Oil [Member]	Sep. 30, 2013 Collars Crude Oil [Member]	Sep. 30, 2014 Collars Natural Gas [Member]	Sep. 30, 2014 Collars Natural Gas [Member]	Dec. 31, 2014 Subsequent Event [Member]
Derivatives, Fair Value [Line Items]
Cash Proceeds from Liquidated Derivatives																			$ 433,000,000
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	190,000	(40,349,000)	(97,217,000)	(51,910,000)	(4,126,000)	(39,298,000)	(77,148,000)	(46,810,000)	4,549,000	14,030,000	(17,799,000)	9,601,000	(233,000)	(15,081,000)	(2,270,000)	(14,701,000)
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	473,809,000	(163,425,000)	269,018,000	(37,638,000)	416,637,000	(146,782,000)	228,845,000	(38,234,000)	25,851,000	(3,400,000)	7,944,000	11,633,000	27,386,000	(13,243,000)	28,300,000	(11,037,000)	3,935,000	3,929,000
Gain (loss) on derivative instruments, net	$ 473,999,000	$ (203,774,000)	$ 171,801,000	$ (89,548,000)