Derivative Instruments and Hedging Activities (Assets and Liabilities - Current vs. Noncurrent Derivative instruments) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative Asset, Current		$ 84	$ 99
Derivative Liability, Current		211	155
Derivative Asset, Noncurrent		264	277
Derivative Liability, Noncurrent		104	269
Total asset derivatives		348	376
Total liability derivatives		315	424
Contracts Subject To Netting Arrangements
Derivatives, Fair Value [Line Items]
Total liability derivatives	[1]	15	41
Collateral Already Posted, Aggregate Fair Value	[1]	$ 9	$ 18

[1] Based on the credit rating of certain subsidiaries