Debt and Other Financing Debt and Other Financing (Schedule of Public Notes) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Six Point Five Percent Notes, Due March Two Thousand Nineteen [Member]
Debt Instrument [Line Items]
Remaining Principal	$ 237.9	$ 237.9
Unamortized Discounts	(0.4)	(0.7)
Unamortized Debt Issuance Costs	(0.3)	(0.4)
Total	237.2	236.8
Four Point Six Zero Percent Notes, Due March Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Remaining Principal	409.9	409.9
Unamortized Discounts	(0.2)	(0.3)
Unamortized Debt Issuance Costs	(0.9)	(1.4)
Total	408.8	408.2
Five Point Zero Percent Notes, Due March Two Thousand Twenty-Three [Member]
Debt Instrument [Line Items]
Remaining Principal	488.9	488.9
Unamortized Discounts	(2.5)	(2.9)
Unamortized Debt Issuance Costs	(1.9)	(2.3)
Total	484.5	483.7
Six Point Nine Five Percent Notes, Due March Two Thousand Forty-Three [Member]
Debt Instrument [Line Items]
Remaining Principal	243.8	243.8
Unamortized Discounts	(0.6)	(0.6)
Unamortized Debt Issuance Costs	(2.2)	(2.4)
Total	$ 241.0	$ 240.8