Debt (Schedule of Revolving Credit Facility) (Details) - Revolving Credit Facility [Member] - USD ($) $ in Millions			3 Months Ended
		Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Borrowings outstanding	[1]	$ 200	$ 200	$ 0
Senior Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Proceeds from Lines of Credit			250
Repayments of Lines of Credit			50
Borrowings outstanding	[2]	200	200
Letters of credit issued		0	0
Available for borrowing	[3]	$ 800	$ 800
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 2.6% as of June 30, 2017.

[2] The Company borrowed $250 million and repaid $50 million under its senior unsecured revolving credit facility during the three months ended June 30, 2017.

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