Risk Management Fair Value of Derivative Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	$ 450	$ 471
Liability derivatives	(148)	(169)
Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a)
Derivatives, Fair Value [Line Items]
Asset derivatives	79	171
Liability derivatives	(77)	(81)
Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Fair Value of Derivatives Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	65	101
Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	(53)	(57)
Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	14	70
Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Other Long-Term Liabilities and Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	(24)	(24)
Designated as Hedging Instrument [Member] | Interest rate swap agreements
Derivatives, Fair Value [Line Items]
Asset derivatives	205	300
Liability derivatives	(65)	(57)
Designated as Hedging Instrument [Member] | Interest rate swap agreements | Fair Value of Derivatives Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	41	94
Designated as Hedging Instrument [Member] | Interest rate swap agreements | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	(3)	0
Designated as Hedging Instrument [Member] | Interest rate swap agreements | Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	164	206
Designated as Hedging Instrument [Member] | Interest rate swap agreements | Other Long-Term Liabilities and Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	(62)	(57)
Designated as Hedging Instrument [Member] | Currency Swap [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	166	0
Liability derivatives	(6)	(31)
Designated as Hedging Instrument [Member] | Currency Swap [Member] | Fair Value of Derivatives Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	0	0
Designated as Hedging Instrument [Member] | Currency Swap [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	(6)	(7)
Designated as Hedging Instrument [Member] | Currency Swap [Member] | Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	166	0
Designated as Hedging Instrument [Member] | Currency Swap [Member] | Other Long-Term Liabilities and Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	0	(24)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	458	474
Liability derivatives	(172)	(199)
Not Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Fair Value of Derivatives Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	8	3
Not Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	(22)	(29)
Not Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	0	0
Not Designated as Hedging Instrument [Member] | Energy commodity derivative contracts(a) | Other Long-Term Liabilities and Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives	(2)	(1)
Not Designated as Hedging Instrument [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives	8	3
Liability derivatives	$ (24)	$ (30)