Impairments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of property	$ 34,000	$ 71,000	$ 9,700	$ 35,100	$ 104,688	$ 43,067	$ 102,332
Arkoma Divestiture Restructuring
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of property		71,000			71,100
Appalachian
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of property					26,600
Mississippi
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of property					$ 7,000