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Commitments And Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 1 Months Ended | 12 Months Ended |  |
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2011 Fracturing Services [Member] | Dec. 31, 2011 Drilling Contracts [Member] | Jan. 31, 2012 Long Term Commitments [Member] | Dec. 31, 2011 Long Term Commitments [Member] | Jan. 31, 2012 Millions Of Barrels [Member] Long Term Commitments [Member] | Dec. 31, 2011 Millions Of Barrels [Member] Long Term Commitments [Member] Williston Basin Project [Member] Volume Commitment Agreements [Member] | Dec. 31, 2011 Billions of Cubic Feet [Member] Long Term Commitments [Member] Williston Basin Project [Member]
Supply Commitment [Line Items] |  |  |  |  |  |  |  |  |  |
Rental expense | $ 0.8 | $ 0.6 | $ 0.4 |  |  |  |  |  |  |
Payments upon early contract termination |  |  |  | 102.4 | 63.2 |  |  |  |  |
Minimum quantity of delivery |  |  |  |  |  |  |  | 4.4 | 9.6 | 18.2
Future obligations under supply agreements |  |  |  |  |  | $ 13.0 | $ 54.5 |  |  |

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