Derivative Instruments - Additional Information (Detail) (USD $)					12 Months Ended		1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Commodity contracts Entity	Dec. 31, 2011 Commodity contracts Credit Concentration Risk Entity	Dec. 31, 2011 Commodity contracts Natural Gas	Dec. 31, 2011 Commodity contracts Crude Oil	Aug. 31, 2011 10.375% Senior Unsecured Notes due 2016	Dec. 31, 2009 10.375% Senior Unsecured Notes due 2016
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Third-party quoted forward prices					NYMEX Henry Hub natural gas	West Texas Intermediate crude oil
Entered interest rate swap								to establish variable rates on approximately one-third of the face amount of the outstanding obligation
Cash proceeds from termination of interest rate swap							$ 2,900,000
Derivative assets	$ 18,987,000	$ 20,707,000	$ 19,000,000
Number of commodity derivative counterparties			6	3