Income Taxes (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Reduction of tax provision			$ 464.0
Transition tax expense		$ 0.3
Increase (decrease) in income tax provision	$ (473.0)	(3.7)
Income taxes paid, net of refunds received		210.0	370.0	$ 265.0
Net operating loss carryforwards		118.0
Unrecognized tax benefits if recognized that would impact effective tax rate	31.0	96.3	31.0
Unrecognized tax benefits if recognized that would not impact effective tax rate		63.0
Unrecognized tax benefits, increase resulting from prior period tax positions		75.0	1.7	0.1
Interest on income taxes accrued		2.0	3.0	1.0
Accrued liability for penalties	0.5	0.5	0.5	0.5
Accrued liability for interest related to uncertain tax positions and penalties	13.2	11.7	13.2	11.6
Accounting Standards Update 2016-09
Income Tax Contingency [Line Items]
Increase (decrease) in income tax provision		(12.1)	(19.6)
Various Tax Credits, Resolution of Tax Matters, and Change in Tax Rates
Income Tax Contingency [Line Items]
Increase (decrease) in income tax provision		(6.4)		(13.0)
Investment Tax Credit Carryforward
Income Tax Contingency [Line Items]
Increase (decrease) in income tax provision		(50.0)	(13.0)	(10.0)
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Increase (decrease) in income tax provision	4.0	4.0
State and Local Jurisdiction | SEC Schedule, 12-09, Valuation Allowance, Operating Loss Carryforward
Income Tax Contingency [Line Items]
Net operating loss carryforwards		73.0
Internal Revenue Service (IRS)
Income Tax Contingency [Line Items]
Increase (decrease) in income tax provision	$ 4.0
Gross reduction in unrecognized tax benefits		$ 9.5	$ 8.5	$ 1.0