Lines of Credit and Financing Arrangements (Domestic Credit Facility) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Nov. 13, 2014
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 650,000,000
Subfacility for Swingline Loans [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 65,000,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Minimum principal amount accelerated	75,000,000
Medium-term Notes [Member] | Domestic Credit Facility [Member]
Line of Credit Facility [Line Items]
Face amount of debt	205,000,000	$ 250,000,000
Periodic payments	7,500,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings	701,000,000
Committed standby letters of credit	2,900,000
Available for future borrowings	151,100,000
Revolving Credit Facility [Member] | Domestic Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum increase in borrowings	$ 350,000,000