Income Taxes (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred Tax Liability Not Recognized [Line Items]
Property, plant and equipment	$ (3,389)		$ (3,131)
Securitized regulatory assets	(256)		(360)
Alternative minimum tax credit carry-forwards	254		294
Merger basis differences	42		50
Pension and benefits	(33)		(39)
Other comprehensive income	101		99
Derivative assets liabilities	66		64
State net operating loss and credit carry-forwards	37		30
Other	41		(45)
Deferred Tax Assets Liabilities Gross	(3,137)		(3,038)
Less valuation allowance	(33)		(27)
Deferred Tax Assets (Liabilities), Net	(3,170)		(3,065)
Current deferred income tax assets	21	51	51
Long-term deferred income tax liabilities	(3,191)	(3,116)	(3,116)
Deferred income tax assets	1,038		1,048
Deferred income tax liabilities	$ (4,208)		$ (4,113)