Lines of Credit and Financing Arrangements (Asset Securitization Program) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Asset Securitization Borrowing Capacity	$ 0	$ 14,000,000
Maximum securitization as percentage of net pool balance	100.00%
Program fee percentage	0.70%
Average floating commercial paper rate (as a percent)	2.63%	2.60%
Unused fee (as a percent)	101.00%
Fixed rate of agreement (as a percent)	0.35%
Senior Unsecured Notes [Member]
Line of Credit Facility [Line Items]
Minimum principal amount accelerated	$ 75,000,000.0
Minimum [Member]
Line of Credit Facility [Line Items]
Asset Securitization Borrowing Capacity	225,000,000
Maximum [Member]
Line of Credit Facility [Line Items]
Asset Securitization Borrowing Capacity	$ 380,000,000