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Income Taxes Schedule of Income Tax Expense (Benefit) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Federal Income Tax Expense (Benefit), Continuing Operations [Abstract]      
Current Federal Tax Expense (Benefit) $ (324.0)us-gaap_CurrentFederalTaxExpenseBenefit $ (92.2)us-gaap_CurrentFederalTaxExpenseBenefit $ (10.3)us-gaap_CurrentFederalTaxExpenseBenefit
Deferred Federal Income Tax Expense (Benefit) 110.3us-gaap_DeferredFederalIncomeTaxExpenseBenefit 152.3us-gaap_DeferredFederalIncomeTaxExpenseBenefit 15.6us-gaap_DeferredFederalIncomeTaxExpenseBenefit
State and Local Income Tax Expense (Benefit), Continuing Operations [Abstract]      
Current State and Local Tax Expense (Benefit) (15.5)us-gaap_CurrentStateAndLocalTaxExpenseBenefit (1.4)us-gaap_CurrentStateAndLocalTaxExpenseBenefit (1.8)us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Deferred State and Local Income Tax Expense (Benefit) (3.3)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit 1.4us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit (5.4)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
10% annual discount for estimated timing of net cash flows 5,159.9us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows 4,680.2us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows 3,942.0us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows
Future production costs 9,842.1us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts 8,400.3us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts 5,027.2us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts
Future net cash flows 10,499.9us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesNetCashFlows 9,064.1us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesNetCashFlows 6,976.7us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesNetCashFlows
Future income tax expenses 4,304.0us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense 3,284.6us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense 2,269.0us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense
Income tax (provision) benefit $ (232.5)us-gaap_IncomeTaxExpenseBenefit $ 60.1us-gaap_IncomeTaxExpenseBenefit $ (1.9)us-gaap_IncomeTaxExpenseBenefit