Significant Accounting Policies, Regulatory Assets Liability (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Regulatory Assets [Line Items]
Regulatory Assets	$ 631	$ 545	$ 550
Regulatory Liabilities [Line Items]
Regulatory Liabilities	783	636	626
Recoverable regulatory infrastructure program costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	331	292	287
Recoverable regulatory infrastructure program costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	257	228	224
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	220	171	161
Environmental Restoration Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	178	132	125
Recoverable seasonal rates [Member]
Regulatory Assets [Line Items]
Regulatory Assets	9	11	10
Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	9	9	9
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets	44	42	53
Total Regulatory Assets [Member]
Regulatory Assets [Line Items]
Regulatory Assets	613	525	520
Derivative financial instruments [Member]
Regulatory Assets [Line Items]
Regulatory Assets	18	20	30
Derivative financial instruments [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	18	20	30
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	254	182	187
Deferred natural gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	26	23	9
Deferred Income Tax Charges [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	14	15	16
Unamortized investment tax credit [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	11	12	12
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	25	24	23
Total Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	348	276	277
Total Associated Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 435	$ 360	$ 349