Selected Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unaudited quarterly financial information
Oil and natural gas revenues	$ 52,748	$ 38,008	$ 36,078	$ 29,164	$ 14,991	$ 17,447	$ 20,864	$ 13,698	$ 155,998	$ 67,000	$ 34,042
Realized gain on derivatives	2,813	3,371	4,713	3,063	2,869	1,435	952	1,850	13,960	7,106	5,299
Unrealized (loss) gain on derivatives	(3,653)	(12,993)	15,114	(3,270)	3,604	2,870	332	(1,668)	(4,802)	5,138	3,139
Expenses	72,377	38,087	66,263	21,857	15,784	15,469	14,953	48,347	198,584	94,552	32,719
Other expense	(907)	(89)	(31)	(235)	(309)	(89)	(89)	(35)	(1,263)	(522)	137
(Loss) income before income taxes	(21,376)	(9,790)	(10,389)	6,865	5,371	6,194	7,106	(34,502)	(34,691)	(15,830)	9,898
Income tax (benefit) provision	(188)	(593)	(3,713)	3,064	1,430		(46)	(6,906)	(1,430)	(5,521)	3,521
Net (loss) income	$ (21,188)	$ (9,197)	$ (6,676)	$ 3,801	$ 3,941	$ 6,194	$ 7,152	$ (27,596)	$ (33,261)	$ (10,309)	$ 6,377
Class A [Member]
Basic
Basic	$ (0.38)	$ (0.17)	$ (0.12)	$ 0.08	$ 0.09	$ 0.14	$ 0.17	$ (0.65)	$ (0.62)	$ (0.25)	$ 0.15
Diluted
Diluted	$ (0.38)	$ (0.17)	$ (0.12)	$ 0.08	$ 0.09	$ 0.14	$ 0.17	$ (0.65)	$ (0.62)	$ (0.25)	$ 0.15
Class B [Member]
Basic
Basic	$ 0.00	$ 0.00	$ 0.00	$ 0.15	$ 0.16	$ 0.21	$ 0.23	$ (0.58)	$ (0.35)	$ 0.02	$ 0.42
Diluted
Diluted	$ 0.00	$ 0.00	$ 0.00	$ 0.15	$ 0.16	$ 0.21	$ 0.23	$ (0.58)	$ (0.35)	$ 0.02	$ 0.42