Debt and Other Financing (Schedule of Public Notes) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
6.50% Notes, due March 2019
Debt Instrument [Line Items]
Remaining Principal	$ 0.0	$ 237.9
Unamortized Discounts	0.0	(0.4)
Unamortized Debt Issuance Costs	0.0	(0.3)
Total	0.0	237.2
4.60% Notes, due March 2020
Debt Instrument [Line Items]
Remaining Principal	387.0	409.9
Unamortized Discounts	(0.1)	(0.2)
Unamortized Debt Issuance Costs	(0.5)	(0.9)
Total	386.4	408.8
5.00% Notes, due March 2023
Debt Instrument [Line Items]
Remaining Principal	461.9	488.9
Unamortized Discounts	(1.9)	(2.5)
Unamortized Debt Issuance Costs	(1.5)	(1.9)
Total	458.5	484.5
6.95% Notes, due March 2043
Debt Instrument [Line Items]
Remaining Principal	243.9	243.8
Unamortized Discounts	(0.6)	(0.6)
Unamortized Debt Issuance Costs	(2.2)	(2.2)
Total	$ 241.1	$ 241.0