Long-Term Debt Long-Term Debt (Schedule of Revolving Credit Facility) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Outstanding borrowings	$ 1,085.7	$ 938.6
Outstanding letters of credit	31.7
Revolving Credit Facility
Debt Instrument [Line Items]
Outstanding borrowings	259.5	75.9
Total credit under facility	700.0	700.0
Undrawn availability	$ 326.2	$ 399.0
Interest rate	2.80%	2.50%
Outstanding letters of credit	$ 24.6	$ 21.7
Original issuance	Jul. 31, 2011	Jul. 31, 2011
Maturity date	Sep. 30, 2018	May 31, 2022