Debt Debt (Schedule of Revolving Credit Facility) (Details) - Revolving Credit Facility [Member] - USD ($) $ in Millions				9 Months Ended
		Sep. 30, 2016	Jul. 15, 2016	Sep. 30, 2016	Dec. 31, 2015
Senior Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Borrowings outstanding	[1]	$ 0		$ 0
Letters of credit issued		0		0
Available for borrowing	[2]	1,000		1,000
Proceeds from lines of credit			$ 409	409
Repayments of lines of credit				411
Senior Secured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Borrowings outstanding	[3]	$ 0		0	$ 350
Proceeds from lines of credit				245
Repayments of lines of credit				$ 595
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 Company borrowed $409 million and repaid $411 million under its new senior unsecured revolving credit facility during the three months ended September 30, 2016. The Company borrowed $245 million and repaid $595 million under its previous secured revolving credit facility during the nine months ended September 30, 2016.

[2] The margin for borrowings under the senior unsecured revolving credit facility was 1.5% above LIBOR.

[3] The weighted average interest rate was 1.8% as of December 31, 2015.