Debt (Schedule of Revolving Credit Facility) (Details) - Revolving Credit Facility [Member] - USD ($) $ in Millions			9 Months Ended
		Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Borrowings outstanding	[1]	$ 0	$ 0	$ 97
Senior Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Proceeds from Lines of Credit			640
Repayments of Lines of Credit			737
Borrowings outstanding	[2]	0	0
Letters of credit issued		0	0
Available for borrowing	[3]	$ 1,000	$ 1,000
London Interbank Offered Rate (LIBOR) [Member] | Senior Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.50%

[1] The weighted average interest rate was 4.1% as of December 31, 2017.

[2] The Company borrowed $640 million and repaid $737 million under its senior unsecured revolving credit facility during the nine months ended September 30, 2018.

[3] The margin for borrowings under the senior unsecured revolving credit facility was 1.5% above LIBOR at current Company credit ratings.