Income Taxes Components of Net Deferred Income Tax Liability (Details) (USD $) In Millions, unless otherwise specified			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Maximum [Member]
Deferred income tax assets:
Regulatory liabilities	$ 346	$ 339
Employee benefits	219	212
Derivative contracts	46	100
Unamortized contract values	63	72
State carryforwards	69	62
Other	213	155
Total deferred income tax assets	956	940
Deferred income tax liabilities:
Property, plant and equipment	(4,269)	(3,919)
Regulatory assets	(695)	(715)
Other	(48)	(40)
Total deferred income tax liabilities	(5,012)	(4,674)
Net deferred income tax liability	(4,056)	(3,734)
Reflected as:
Deferred income taxes - current assets	112	129
Deferred income taxes - noncurrent liabilities	$ (4,168)	$ (3,863)
State carryforwards expiration date			2013	2032