Derivative Instruments - Additional Information (Detail) (USD $)			1 Months Ended					9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	May 31, 2012 7.25% Senior Unsecured Notes due 2019	Sep. 30, 2013 7.25% Senior Unsecured Notes due 2019	Feb. 29, 2012 7.25% Senior Unsecured Notes due 2019	Sep. 30, 2013 Commodity contracts Entity	Sep. 30, 2013 Commodity contracts Credit Concentration Risk Entity	Sep. 30, 2013 Commodity contracts Natural Gas	Sep. 30, 2013 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
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	$ 6,324,000	$ 16,473,000				$ 6,300,000
Number of derivative counterparty						6	3