Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Debt securities:
Domestic government	$ 11	$ 12
Corporate credit	8	8
Money market funds	22	21
Non-qualified benefit plan trust: (2)
Fair Value - Money market funds	1	1
Domestic	6	4
Debt securities—domestic government	1	1
Investments, Fair Value Disclosure	0	2
Alternative Investments, Fair Value Disclosure		0
Assets from price risk management activities: (1) (3)
Electricity	5	7
Natural gas	6	16
Total	22	23
Assets, Fair Value Disclosure	60	72
Liabilities from price risk management activities: (1) (3)
Electricity	149	118
Natural gas	66	51
Total	215	169
Fair Value, Inputs, Level 1 [Member]
Debt securities:
Domestic government	2	2
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Fair Value - Money market funds	1	1
Domestic	6	4
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	10	8
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	10
Corporate credit	8	8
Non-qualified benefit plan trust: (2)
Fair Value - Money market funds		0
Domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	3	6
Natural gas	6	15
Total	26	39
Liabilities from price risk management activities: (1) (3)
Electricity	16	6
Natural gas	53	42
Total	69	48
Fair Value, Inputs, Level 3 [Member]
Debt securities:
Domestic government	0	0
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Fair Value - Money market funds		0
Domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	2	1
Natural gas	0	1
Total	2	2
Liabilities from price risk management activities: (1) (3)
Electricity	133	112
Natural gas	13	9
Total	$ 146	$ 121