Fair Value (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 112	$ 123
Assets [Abstract]
Nuclear decommissioning trusts	1,080	1,037
Other investments	119	110
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	876	1,016
Assets, Fair Value Disclosure, Recurring	1,428	1,417
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(870)	(1,020)
Derivative Liabilities	(106)	(151)
Derivative Assets (Liabilities), at Fair Value, Net	1,322	1,266
Derivative Assets, Current	90	108
Derivative Assets, Noncurrent	27	39
Derivative Liabilities, Current	90	125
Derivative Liabilities, Noncurrent	16	26
Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Current	202	231
Non Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Noncurrent	1,226	1,186
Current Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Current	(90)	(125)
Noncurrent Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Noncurrent	(16)	(26)
Interest rate contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	(1)
Natural Gas [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	14	40
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(41)	(56)
Electricity [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	97	102
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(60)	(93)
Other [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	6	5
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(4)	(1)
Total derivative assets [Member]
Derivative assets [Abstract]
Derivative Assets	117	147
Foreign currency exchange contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(1)	0
Total derivative liabilities [Member]
Derivative liabilities [Abstract]
Derivative Liabilities	(106)	(151)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Assets [Abstract]
Nuclear decommissioning trusts	749	694
Other investments	72	66
Derivative assets [Abstract]
Assets, Fair Value Disclosure, Recurring	1,231	1,321
Derivative liabilities [Abstract]
Derivative Liabilities	(409)	(532)
Derivative Assets (Liabilities), at Fair Value, Net	822	789
Fair Value, Inputs, Level 1 [Member] | Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Current	379	493
Fair Value, Inputs, Level 1 [Member] | Non Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Noncurrent	852	828
Fair Value, Inputs, Level 1 [Member] | Current Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Current	(368)	(466)
Fair Value, Inputs, Level 1 [Member] | Noncurrent Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Noncurrent	(41)	(66)
Fair Value, Inputs, Level 1 [Member] | Interest rate contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Fair Value, Inputs, Level 1 [Member] | Natural Gas [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	365	555
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(362)	(526)
Fair Value, Inputs, Level 1 [Member] | Electricity [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Fair Value, Inputs, Level 1 [Member] | Other [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	45	6
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(47)	(6)
Fair Value, Inputs, Level 1 [Member] | Total derivative assets [Member]
Derivative assets [Abstract]
Derivative Assets	410	561
Fair Value, Inputs, Level 1 [Member] | Foreign currency exchange contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Fair Value, Inputs, Level 1 [Member] | Total derivative liabilities [Member]
Derivative liabilities [Abstract]
Derivative Liabilities	(409)	(532)
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	112	123
Assets [Abstract]
Nuclear decommissioning trusts	331	343
Other investments	47	44
Derivative assets [Abstract]
Assets, Fair Value Disclosure, Recurring	824	805
Derivative liabilities [Abstract]
Derivative Liabilities	(313)	(315)
Derivative Assets (Liabilities), at Fair Value, Net	511	490
Fair Value, Inputs, Level 2 [Member] | Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Current	389	372
Fair Value, Inputs, Level 2 [Member] | Non Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Noncurrent	435	433
Fair Value, Inputs, Level 2 [Member] | Current Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Current	(263)	(269)
Fair Value, Inputs, Level 2 [Member] | Noncurrent Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Noncurrent	(50)	(46)
Fair Value, Inputs, Level 2 [Member] | Interest rate contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	(1)
Fair Value, Inputs, Level 2 [Member] | Natural Gas [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	107	66
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(86)	(73)
Fair Value, Inputs, Level 2 [Member] | Electricity [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	219	226
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(221)	(240)
Fair Value, Inputs, Level 2 [Member] | Other [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	8	3
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(5)	(1)
Fair Value, Inputs, Level 2 [Member] | Total derivative assets [Member]
Derivative assets [Abstract]
Derivative Assets	334	295
Fair Value, Inputs, Level 2 [Member] | Foreign currency exchange contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(1)	0
Fair Value, Inputs, Level 2 [Member] | Total derivative liabilities [Member]
Derivative liabilities [Abstract]
Derivative Liabilities	(313)	(315)
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Assets [Abstract]
Nuclear decommissioning trusts	0	0
Other investments	0	0
Derivative assets [Abstract]
Assets, Fair Value Disclosure, Recurring	132	160
Derivative liabilities [Abstract]
Derivative Liabilities	(148)	(173)
Derivative Assets (Liabilities), at Fair Value, Net	(16)	(13)
Fair Value, Inputs, Level 3 [Member] | Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Current	101	120
Fair Value, Inputs, Level 3 [Member] | Non Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Noncurrent	31	40
Fair Value, Inputs, Level 3 [Member] | Current Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Current	(119)	(144)
Fair Value, Inputs, Level 3 [Member] | Noncurrent Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Noncurrent	(29)	(29)
Fair Value, Inputs, Level 3 [Member] | Interest rate contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Fair Value, Inputs, Level 3 [Member] | Natural Gas [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	32	24
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(62)	(62)
Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	98	134
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(86)	(111)
Fair Value, Inputs, Level 3 [Member] | Other [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	2	2
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Fair Value, Inputs, Level 3 [Member] | Total derivative assets [Member]
Derivative assets [Abstract]
Derivative Assets	132	160
Fair Value, Inputs, Level 3 [Member] | Foreign currency exchange contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Fair Value, Inputs, Level 3 [Member] | Total derivative liabilities [Member]
Derivative liabilities [Abstract]
Derivative Liabilities	(148)	(173)
Netting Adjustments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Assets [Abstract]
Nuclear decommissioning trusts	0	0
Other investments	0	0
Derivative assets [Abstract]
Assets, Fair Value Disclosure, Recurring	(759)	(869)
Derivative liabilities [Abstract]
Derivative Liabilities	764	869
Derivative Assets (Liabilities), at Fair Value, Net	5	0
Netting Adjustments [Member] | Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Current	(667)	(754)
Netting Adjustments [Member] | Non Current Asset [Member]
Derivative liabilities [Abstract]
Derivative Assets, Noncurrent	(92)	(115)
Netting Adjustments [Member] | Current Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Current	660	754
Netting Adjustments [Member] | Noncurrent Liability [Member]
Derivative liabilities [Abstract]
Derivative Liabilities, Noncurrent	104	115
Netting Adjustments [Member] | Interest rate contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Netting Adjustments [Member] | Natural Gas [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	(490)	(605)
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	469	605
Netting Adjustments [Member] | Electricity [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	(220)	(258)
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	247	258
Netting Adjustments [Member] | Other [Member]
Derivative assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	(49)	(6)
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	48	6
Netting Adjustments [Member] | Total derivative assets [Member]
Derivative assets [Abstract]
Derivative Assets	(759)	(869)
Netting Adjustments [Member] | Foreign currency exchange contracts [Member]
Derivative liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Netting Adjustments [Member] | Total derivative liabilities [Member]
Derivative liabilities [Abstract]
Derivative Liabilities	$ 764	$ 869