Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current Income Tax Expense [Abstract]
Federal	$ 160	$ 80	$ 105
State	22	19	17
Total current expense	182	99	122
Deferred Income Tax Expense (Benefit) [Abstract]
Federal	(69)	146	(5)
State	0	3	(1)
Total deferred expense (benefit)	(69)	149	(6)
Total Income Tax Expense	113	248	116
Income Tax Reconciliation [Abstract]
Income before income taxes and extraordinary item	392	736	316
Federal statutory income tax rate	35.00%	35.00%	35.00%
Expected federal income tax expense	137	258	111
Increase (decrease) in tax expense resulting from:
State income tax expense, net of federal income tax	14	15	10
Amortization of investment tax credit	(1)	(6)	(7)
Increase (decrease) in settled and uncertain tax positions	(26)	(4)	9
Other, net	(11)	(15)	(7)
Total	(24)	(10)	5
Total Income Tax Expense	113	248	116
Effective tax rate	28.80%	33.70%	36.70%
Amount of reduction to regulatory assets due to actuarially determined net present value of lost future health care deductions			4
Reduction In Liability For Uncertain Tax Positions	26
Investment Tax Credit	1
Deferred Tax Assets and Liabilities [Abstract]
Allowance for doubtful accounts	1	1
Other, current	0	2
Total current deferred tax assets	1	3
Employee benefits	95	94
Other, non-current	9	10
Total non-current deferred tax assets	104	104
Total deferred tax assets	105	107
Depreciation	1,103	943
Regulatory assets, net	1,098	1,274
Total deferred tax liabilities	2,201	2,217
Accumulated deferred income taxes, net	2,096	2,110
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, beginning of year	44	232	175
Unrecognized tax benefits related to prior years [Abstract]
Additions	0	0	10
Reductions	(46)	(192)	(3)
Unrecognized tax positions related to current year [Abstract]
Additions	0	4	50
Settlements	2	0	0
Unrecognized Tax Benefits, end of year	0	44	232
Unrecognized tax benefits that would reduce effective tax rate		18	14
Unrecognized tax benefits interest on income tax expense	(5)	(12)	8
Unrecognized tax benefits interest on income taxes accrued		5
Amount of reduction to deferred tax assets due to actuarially determined net present value of lost future health care deductions			$ 7