Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Estimate of Fair Value, Fair Value Disclosure [Member]
Nuclear decommissioning trust (1):
Money market funds	$ 14	$ 14
Debt securities:
Domestic	15	12
Corporate credit	8	11
Equity securities:
Domestic	7	9
International	1	1
Debt securities - domestic government	2	3
Assets from price risk management activities (1) (3):
Electricity	5	2
Natural gas	3	17
Total	55	69
Liabilities from price risk management activities:
Electricity	116	137
Natural gas	197	251
Total	313	388
Fair Value, Inputs, Level 1 [Member]
Nuclear decommissioning trust (1):
Money market funds	0	0
Debt securities:
Domestic	4	3
Corporate credit	0	0
Equity securities:
Domestic	5	7
International	1	1
Debt securities - domestic government	2	3
Assets from price risk management activities (1) (3):
Electricity	0	0
Natural gas	0	0
Total	12	14
Liabilities from price risk management activities:
Electricity	0	0
Natural gas	0	0
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Nuclear decommissioning trust (1):
Money market funds	14	14
Debt securities:
Domestic	11	9
Corporate credit	8	11
Equity securities:
Domestic	2	2
International	0	0
Debt securities - domestic government	0	0
Assets from price risk management activities (1) (3):
Electricity	5	2
Natural gas	2	17
Total	42	55
Liabilities from price risk management activities:
Electricity	85	108
Natural gas	139	201
Total	224	309
Fair Value, Inputs, Level 3 [Member]
Nuclear decommissioning trust (1):
Money market funds	0	0
Debt securities:
Domestic	0	0
Corporate credit	0	0
Equity securities:
Domestic	0	0
International	0	0
Debt securities - domestic government	0	0
Assets from price risk management activities (1) (3):
Electricity	0	0
Natural gas	1	0
Total	1	0
Liabilities from price risk management activities:
Electricity	31	29
Natural gas	58	50
Total	$ 89	$ 79