Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Estimate of Fair Value, Fair Value Disclosure [Member]
Nuclear decommissioning trust (1):
Money market funds	$ 59	$ 15
Debt securities:
Domestic government	15	15
Corporate credit	8	8
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		2
Equity securities:
Domestic	8	4
International		1
Debt securities - domestic government	1	2
Assets from price risk management activities:
Electricity		1
Natural gas	1	5
Total	92	53
Liabilities from price risk management activities: (1) (3)
Electricity	78	82
Natural gas	82	118
Total	160	200
Fair Value, Inputs, Level 1 [Member]
Nuclear decommissioning trust (1):
Money market funds	0	0
Debt securities:
Domestic government	6	7
Corporate credit	0	0
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		0
Equity securities:
Domestic	5	2
International		1
Debt securities - domestic government	1	2
Assets from price risk management activities:
Electricity		0
Natural gas	0	0
Total	12	12
Liabilities from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Nuclear decommissioning trust (1):
Money market funds	59	15
Debt securities:
Domestic government	9	8
Corporate credit	8	8
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		2
Equity securities:
Domestic	3	2
International		0
Debt securities - domestic government	0	0
Assets from price risk management activities:
Electricity		1
Natural gas	1	3
Total	80	39
Liabilities from price risk management activities: (1) (3)
Electricity	39	72
Natural gas	57	110
Total	96	182
Fair Value, Inputs, Level 3 [Member]
Nuclear decommissioning trust (1):
Money market funds	0	0
Debt securities:
Domestic government	0	0
Corporate credit	0	0
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		0
Equity securities:
Domestic	0	0
International		0
Debt securities - domestic government	0	0
Assets from price risk management activities:
Electricity		0
Natural gas	0	2
Total	0	2
Liabilities from price risk management activities: (1) (3)
Electricity	39	10
Natural gas	25	8
Total	$ 64	$ 18