Income Taxes - Components of Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current tax expense (benefit):
Federal	$ 87.0us-gaap_CurrentFederalTaxExpenseBenefit	$ 84.6us-gaap_CurrentFederalTaxExpenseBenefit	$ 51.3us-gaap_CurrentFederalTaxExpenseBenefit
State	1.4	(6.4)	(5.3)
Total current tax expense (benefit)	88.4us-gaap_CurrentIncomeTaxExpenseBenefit	78.2us-gaap_CurrentIncomeTaxExpenseBenefit	46.0us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred tax expense (benefit):
Federal	(1,829.6)	(112.5)	(49.4)
State	(11.0)	1.8	(1.8)
Total deferred tax expense (benefit)	(1,840.6)us-gaap_DeferredIncomeTaxExpenseBenefit	(110.7)us-gaap_DeferredIncomeTaxExpenseBenefit	(51.2)us-gaap_DeferredIncomeTaxExpenseBenefit
Total income tax expense (benefit)	$ (1,752.2)us-gaap_IncomeTaxExpenseBenefit	$ (32.5)us-gaap_IncomeTaxExpenseBenefit	$ (5.2)us-gaap_IncomeTaxExpenseBenefit

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations.

Amount of current state and local tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred federal income tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred state and local tax expense (benefit) pertaining to income (loss) from continuing operations.