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Property, Plant and Equipment - Additional Information (Detail) (USD $)
 12 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2010
Roosevelt County, Montana [Member]
acre
Dec. 31, 2010
Richland County, Montana [Member]
acre
Dec. 31, 2012
Unproved property leases [Member]
Dec. 31, 2011
Unproved property leases [Member]
Dec. 31, 2010
Unproved property leases [Member]
Property, Plant and Equipment [Line Items]                
Asset retirement costs $ 20,700,000 $ 11,400,000            
Impairment of oil and gas properties 3,581,000 3,610,000 11,967,000     3,600,000 3,600,000 12,000,000
Number of acres acquired       16,700 10,000      
Payments to acquire land       $ 52,300,000 $ 30,100,000