Benefit Obligations (Schedule of Nonqualified Pension Plans Funded with Nonqualified Trusts) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Noncurrent Other assets, consisting of insurance contracts	$ 37	$ 42
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Noncurrent Other assets, consisting of insurance contracts	37	42
Current Other liabilities	21	22
Benefit obligations	213	237
Supplemental Employee Retirement Plan [Member] | Money Market Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Marketable securities, at fair value	$ 31	$ 32