Summary of Significant Accounting Policies - Summary of Revision of Natural Gas, NGL and Oil Sales, Reported (Detail) (USD $) In Thousands, 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
Other revenues, as previously reported	$ 3,085	$ 2,519	$ 5,240	$ 4,597	$ 5,837	$ 3,775	$ 1,506	$ 3,911	$ 15,441	$ 15,029	$ 9,831
Brokered natural gas, marketing and other revenue, reported										15,029	9,831
Revision of brokered natural gas and marketing	4,994	4,887	6,491	4,062	3,151	3,333	2,981	2,521	20,434	11,986	9,761
Other revenues as previously reported [Member]
Other revenues, as previously reported										3,043	70
Revision of brokered natural gas and marketing [Member]
Other revenues, as previously reported										11,986	9,761
Brokered natural gas and marketing previously reported [Member]
Revision of brokered natural gas and marketing
Revision of brokered natural gas and marketing expenses [Member]
Revision of brokered natural gas and marketing										$ 11,986	$ 9,761