Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Sep. 30, 2012	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current income tax expense (benefit):
Federal			$ 0	$ 34	$ (38)
State			1	9	17
Total current expense (benefit)			1	43	(21)
Deferred income tax expense (benefit):
Federal			198	140	234
State			47	4	(26)
Total deferred expense			245	144	208
Total income tax expense			246	187	187
Income tax reconciliation
Income before income taxes			383	503	487
Federal statutory income tax rate			35.00%	35.00%	35.00%
Expected federal income tax expense			134	176	170
Increase (decrease) in tax expense resulting from:
State income tax expense (benefit), net of federal income tax			31	9	(6)
Increase (decrease) in settled and uncertain income tax positions			(7)	1	5
Tax law change in deductibility of retiree health care costs			0	0	18
Goodwill impairment			88	0	0
Other, net			0	1	0
Total			112	11	17
Total income tax expense			246	187	187
Effective tax rate			64.20%	37.20%	38.40%
Goodwill impairment	252		252	0	0
Tax Adjustments, Settlements, and Unusual Provisions			(7)
Amount of reduction to deferred tax assets due to actuarially determined net present value of lost future health care deductions		22
Amount of non-cash recorded to income tax expense as result of change in tax law					19
Amount of reduction to deferred tax assets, recorded to income tax expense, due to actuarially determined net present value of lost future health care deductions		19
Regulatory Assets		1
Amount of decrease in income tax expense recorded related to effects of re-measuring accumulated deferred income taxes					24
Amount of net reduction to income tax expense from state income tax expense, net of federal income tax				20	29
Increase in income tax expense related to uncertain tax positions					5
Deferred Tax Assets [Abstract]
Allowance for Doubtful Accounts			9	9
Other, current deferred tax assets			0	(1)
Total current deferred tax assets			9	8
Employee Benefits			49	57
Loss And Credit Carryforwards			192	238
Regulatory liabilities, net		2	196	0
Other, noncurrent deferred tax assets			51	60
Total non-current deferred tax assets before valuation allowance			488	355
Valuation Allowance			(2)	(3)
Total non-current deferred tax assets, net of valuation allowance			486	352
Total deferred tax assets, net of valuation allowance			495	360
Deferred Tax Liabilities [Abstract]
Deferred Gas Costs			25	0
Other, current deferred tax liabilities			5	0
Total current deferred tax liabilities			30	0
Depreciation			2,033	1,696
Regulatory assets, net			0	17
Other, non-current deferred tax liabilities			129	59
Total non-current deferred tax liabilities			2,162	1,772
Total deferred tax liabilities			2,192	1,772
Accumulated deferred income taxes, net			1,697	1,412
Reconciliation of Unrecognized Tax Benefits (Expenses) [Roll Forward]
Balance, beginning of year		6	8	11	6
Tax Positions Related to Prior Years:
Additions			0	(1)	0
Reductions			(27)	(3)	(2)
Tax Positions Related to Current Year:
Additions			0	1	7
Reductions			(1)	0	0
Balance, end of year			(20)	8	11
Interest expense on uncertain income tax positions			(3)	0	0
Accrued interest receivable on uncertain income tax positions			7	4
Unrecognized Tax Benefits that Would Impact Effective Tax Rate			0	6	5
Federal [Member]
Deferred Tax Liabilities [Abstract]
Federal net operating loss carryforwards to expire in 2030			487
Operating Loss Carryforwards, Expiration Dates			2031
Other Tax Carryforward, Expiration Dates			2013 and 2032
State [Member]
Deferred Tax Liabilities [Abstract]
Operating Loss Carryforwards, Expiration Dates			2013 and 2032
State net operating loss carryforwards to expire in various years between 2012 and 2031			377
Other Tax Carryforward, Expiration Dates			2017
State capital loss carryforwards which expire in 2017			244
State capital loss carryforwards, full valuation allowance			3
State capital loss carryforwards, valuation allowance net of federal tax			2
Capital Loss Carryforward [Member] | Federal [Member]
Deferred Tax Liabilities [Abstract]
Other Tax Carryforward, Gross Amount			1
Charitable Contribution Carryforward [Member] | Federal [Member]
Deferred Tax Liabilities [Abstract]
Other Tax Carryforward, Gross Amount			$ 1