Derivative Instruments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified					6 Months Ended		1 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Credit Concentration Risk Commodity contracts Entity	Jun. 30, 2011 Commodity contracts Entity	Jun. 30, 2011 Commodity contracts Natural Gas	Jun. 30, 2011 Commodity contracts Crude Oil	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
Derivative assets	$ 16,987	$ 20,707		$ 14,300
Number of commodity derivative counterparties			2	4