Debt (Senior Credit Facilities Narrative) (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017	Jul. 15, 2016
Senior Unsecured Term Loan Due 2021 [Member]
Debt Instrument [Line Items]
Senior unsecured debt	[1]	$ 488	$ 494	$ 500
Revolving Credit Facility [Member] | Senior Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing base	[2]			$ 1,000

[1] The margin for borrowings under the senior unsecured term loan due 2021 was 1.5% above LIBOR at current Company credit ratings.

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