Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Debt securities:
Domestic government	$ 13	$ 14
Corporate credit	9	8
Money market funds	19	18
Investments, Fair Value Disclosure	2	2
Non-qualified benefit plan trust: (2)
Domestic	3	3
Debt securities—domestic government	1	1
Alternative Investments, Fair Value Disclosure	1	1
Assets from price risk management activities: (1) (3)
Electricity	9	7
Natural gas	8	3
Total	65	57
Liabilities from price risk management activities: (1) (3)
Electricity	152	133
Natural gas	100	158
Total	252	291
Fair Value, Inputs, Level 1 [Member]
Debt securities:
Domestic government	4	6
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Domestic	3	3
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	8	10
Liabilities from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Debt securities:
Domestic government	9	8
Corporate credit	9	8
Non-qualified benefit plan trust: (2)
Domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	8	7
Natural gas	8	3
Total	34	26
Liabilities from price risk management activities: (1) (3)
Electricity	7	28
Natural gas	86	144
Total	93	172
Fair Value, Inputs, Level 3 [Member]
Debt securities:
Domestic government	0	0
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Domestic	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: (1) (3)
Electricity	145	105
Natural gas	14	14
Total	159	119
Reported Value Measurement [Member]
Debt securities:
Money market funds	19	18
Investments, Fair Value Disclosure	2	2
Non-qualified benefit plan trust: (2)
Alternative Investments, Fair Value Disclosure	1	1
Estimate of Fair Value Measurement [Member]
Assets from price risk management activities: (1) (3)
Total	$ 22	$ 21