Benefit Obligations (Schedule of Nonqualified Pension Plans Funded with Nonqualified Trusts) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Noncurrent Other assets, consisting of insurance contracts	$ 42	$ 49
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Noncurrent Other assets, consisting of insurance contracts	42	49
Current Other liabilities	22	22
Benefit obligations	237	241
Supplemental Employee Retirement Plan [Member] | Money Market Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Marketable securities, at fair value	$ 32	$ 30