Derivative Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended	1 Months Ended		6 Months Ended
	Aug. 31, 2011 10.375% Senior Unsecured Notes due 2016	Mar. 31, 2011 10.375% Senior Unsecured Notes due 2016	May 31, 2012 7.25% Senior Unsecured Notes due 2019	Feb. 29, 2012 7.25% Senior Unsecured Notes due 2019	Jun. 30, 2012 Commodity contracts Natural Gas	Jun. 30, 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
Entered interest rate swap		established variable rates on approximately one-third of the face amount of the outstanding obligation		to establish variable rates on approximately one-third of the outstanding obligation
Cash proceeds from termination of interest rate swap	$ 2.9		$ 1.2