Debt (Senior Credit Facilities Narrative) (Details) - Revolving Credit Facility [Member] - USD ($) $ in Millions				6 Months Ended
		Jun. 30, 2016		Jun. 30, 2016		Dec. 31, 2015
Debt Instrument [Line Items]
Maximum borrowing base		$ 900		$ 900
Borrowings outstanding	[2]	0	[1]	0	[1]	$ 350
Letters of credit issued		0		0
Available for borrowing	[3]	$ 900		900
Proceeds from Lines of Credit				245
Repayments of lines of credit				$ 595
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.50%

[1] The Company borrowed $245 million and repaid $595 million during the six months ended June 30, 2016.

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

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