Lines of Credit and Financing Arrangements (Domestic Credit Facility) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2016	Dec. 31, 2017	Dec. 31, 2015	Nov. 13, 2014
Line of Credit Facility [Line Items]
Proceeds from the facilities			$ 79,000,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity				$ 650,000,000
Domestic Credit Facility [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings		$ 367,000,000
Standby letters of credit		2,900,000
Line of credit, available for future borrowings		500,100,000
Maximum increase in credit facility		350,000,000
Domestic Credit Facility [Member] | Medium-term Notes [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount				$ 250,000,000
Outstanding borrowings		220,000,000
Quarterly principal repayments		7,500,000
Proceeds from the facilities	$ 30,000,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity		100,000,000
Subfacility for Swingline Loans [Member]
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity		65,000,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Minimum amount of principal or interest due on indebtedness or receivable securitization		$ 75,000,000.0