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Commitments And Contingencies (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Supply Commitment [Line Items]  
Total rental commitments under non-cancelable leases $ 12.3
Fracturing Services [Member]
 
Supply Commitment [Line Items]  
Payments upon early contract termination 66.6
Drilling Contracts [Member]
 
Supply Commitment [Line Items]  
Payments upon early contract termination 62.3
Long Term Commitments [Member]
 
Supply Commitment [Line Items]  
Future obligations under supply agreements $ 65.5
Long Term Commitments [Member] | Williston Basin Project [Member]
 
Supply Commitment [Line Items]  
Minimum quantity of delivery 17,600.0
Millions Of Barrels [Member] | Long Term Commitments [Member] | Williston Basin Project [Member]
 
Supply Commitment [Line Items]  
Minimum quantity of delivery 13.1