Derivative Instruments - Additional Information (Detail) (USD $)			1 Months Ended					12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	May 31, 2012 7.25% Senior Unsecured Notes due 2019	Dec. 31, 2012 7.25% Senior Unsecured Notes due 2019	Feb. 29, 2012 7.25% Senior Unsecured Notes due 2019	Dec. 31, 2012 Commodity contracts Entity	Dec. 31, 2012 Commodity contracts Credit Concentration Risk Entity	Dec. 31, 2012 Commodity contracts Natural Gas	Dec. 31, 2012 Commodity contracts Crude Oil
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Third-party quoted forward prices								NYMEX Henry Hub gas	West Texas Intermediate crude oil
Debt hedged by interest rate derivatives					33.33%
Annual interest rate				7.25%	7.25%
Cash proceeds from termination of interest rate swap			$ 1,200,000
Interest rate derivatives outstanding	0
Derivative assets	$ 9,508,000	$ 16,473,000				$ 9,500,000
Number of derivative counterparty						6	3