Benefit Obligations (Schedule of Expense Related to Nonqualified Pension Plans Included in Net Periodic Benefit Cost, Excluding Returns on Assets) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		$ (33)	$ 48	$ 75
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		(1)	1	(3)
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	[1]	$ 18	$ 18	$ 0

[1] Actuarial gain offset interest cost.