Derivatives (Outstanding Derivative Balances) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Other Current Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	$ 47	$ 91
Cash Collateral	9	1
Total Derivative Balance	49	82
Netting	(7)	(10)
Other Noncurrent Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	121	149
Cash Collateral	0	0
Total Derivative Balance	118	140
Netting	(3)	(9)
Other Current Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(54)	(48)
Cash Collateral	11	0
Total Derivative Balance	(36)	(38)
Netting	7	10
Other Noncurrent Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(81)	(101)
Cash Collateral	7	3
Total Derivative Balance	(71)	(89)
Netting	3	9
Gross Derivative Balance [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	33	91
Netting [Member]
Derivatives And Hedging Activities [Line Items]
Netting	0	0
Cash Collateral [Member]
Derivatives And Hedging Activities [Line Items]
Cash Collateral	27	4
Total Derivatve Balance [Member]
Derivatives And Hedging Activities [Line Items]
Total Derivative Balance	$ 60	$ 95