Property and Equipment (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Property and Equipment
Proved properties	$ 2,277,758,000		$ 2,277,758,000		$ 1,522,723,000
Unevaluated properties	466,274,000		466,274,000		313,941,000
Other property and equipment	7,907,000		7,907,000		5,038,000
Less accumulated depreciation, depletion, and amortization	(369,099,000)		(369,099,000)		(274,294,000)
Net property and equipment	2,382,840,000		2,382,840,000		1,567,408,000
Other information
Depletion expense	52,256,000	27,792,000	93,845,000	55,732,000
Depletion expense (per Boe)	29.24	38.65	28.92	37.86
Oil and Gas Properties
Other information
Internal cost capitalized	1,900,000	400,000	3,400,000	400,000
Impairment			$ 0	$ 0
Other Property and Equipment | Minimum
Other information
Estimated useful lives			5 years
Other Property and Equipment | Maximum
Other information
Estimated useful lives			7 years