Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Contingency [Line Items]
Income tax (benefit) expense	$ (1.5)		$ 2.6	$ 69.4	$ 5.9
Deferred income tax expense	$ (9.4)		$ 0.9	70.4	$ 4.5
Federal statutory tax rate (as a percent)			35.00%		35.00%
Effective income tax rate (as a percent)			15.30%		17.40%
IRS
Income Tax Contingency [Line Items]
Deferred income tax expense		$ 148.6
Deferred tax liability		$ 84.4
Net impact on deferred tax expense				$ 64.2