Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Estimate of Fair Value, Fair Value Disclosure [Member]
Nuclear decommissioning trust (1):
Money market funds	$ 14	$ 14
Debt securities:
Domestic	13	12
Corporate credit	9	11
Equity securities:
Domestic	0	9
International	10	1
Debt securities - domestic government	2	3
Assets from price risk management activities (1) (3):
Electricity	7	2
Natural gas	18	17
Total	73	69
Liabilities from price risk management activities:
Electricity	166	137
Natural gas	249	251
Total	415	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	0	7
International	7	1
Debt securities - domestic government	2	3
Assets from price risk management activities (1) (3):
Electricity	0	0
Natural gas	0	0
Total	13	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	9	9
Corporate credit	9	11
Equity securities:
Domestic	0	2
International	3	0
Debt securities - domestic government	0	0
Assets from price risk management activities (1) (3):
Electricity	6	2
Natural gas	18	17
Total	59	55
Liabilities from price risk management activities:
Electricity	130	108
Natural gas	189	201
Total	319	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	1	0
Natural gas	0	0
Total	1	0
Liabilities from price risk management activities:
Electricity	36	29
Natural gas	60	50
Total	$ 96	$ 79