Debt (Schedule of Revolving Credit Facility) (Details) - Revolving Credit Facility [Member] - USD ($) $ in Millions			3 Months Ended
		Mar. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Borrowings outstanding	[1]	$ 197	$ 197	$ 97
Senior Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Proceeds from Lines of Credit			435
Repayments of Lines of Credit			335
Borrowings outstanding	[2]	197	197
Letters of credit issued		0	0
Available for borrowing	[3]	$ 803	$ 803
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 2.4% and 2.1% as of March 31, 2018 and December 31, 2017, respectively.

[2] The Company borrowed $435 million and repaid $335 million under its senior unsecured revolving credit facility during the three months ended March 31, 2018.

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