Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Nuclear decommissioning trust: (1)
Money market funds	$ 17	$ 65
Debt securities:
Domestic government	13	14
Corporate credit	10	11
Equity securities:
Domestic	6	5
International		1
Debt securities—domestic government	1
Assets from price risk management activities: (1) (3)
Electricity	5	5
Natural gas	2	2
Total	54	103
Liabilities from price risk management activities: (1) (3)
Electricity	148	112
Natural gas	151	116
Total	299	228
Fair Value, Inputs, Level 1 [Member]
Nuclear decommissioning trust: (1)
Money market funds	0	0
Debt securities:
Domestic government	5	7
Corporate credit	0	0
Equity securities:
Domestic	4	4
International		1
Debt securities—domestic government	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	10	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	17	65
Debt securities:
Domestic government	8	7
Corporate credit	10	11
Equity securities:
Domestic	2	1
International		0
Debt securities—domestic government	0
Assets from price risk management activities: (1) (3)
Electricity	5	4
Natural gas	2	2
Total	44	90
Liabilities from price risk management activities: (1) (3)
Electricity	31	32
Natural gas	99	95
Total	130	127
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
Equity securities:
Domestic	0	0
International		0
Debt securities—domestic government	0
Assets from price risk management activities: (1) (3)
Electricity	0	1
Natural gas	0	0
Total	0	1
Liabilities from price risk management activities: (1) (3)
Electricity	117	80
Natural gas	52	21
Total	$ 169	$ 101