Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Debt securities:
Domestic government	$ 10	$ 12
Corporate credit	8	8
Money market funds measured at NAV (2)	23	21
Non-qualified benefit plan trust: (2)
Money market funds	2	1
Eqauity securities - domestic	6	4
Debt securities—domestic government	1	1
Collective trust-domestic equity measured at NAV (2)	0	2
Assets from price risk management activities: (1) (3)
Electricity	5	7
Natural gas	2	16
Total	23	23
Assets, Fair Value Disclosure	57	72
Liabilities from price risk management activities: (1) (3)
Electricity	146	118
Natural gas	58	51
Total	204	169
Fair Value, Inputs, Level 1 [Member]
Debt securities:
Domestic government	2	2
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Money market funds	2	1
Eqauity securities - 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	11	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	8	10
Corporate credit	8	8
Non-qualified benefit plan trust: (2)
Money market funds	0	0
Eqauity securities - domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	3	6
Natural gas	2	15
Total	21	39
Liabilities from price risk management activities: (1) (3)
Electricity	2	6
Natural gas	47	42
Total	49	48
Fair Value, Inputs, Level 3 [Member]
Debt securities:
Domestic government	0	0
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Money market funds	0	0
Eqauity securities - 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	144	112
Natural gas	11	9
Total	$ 155	$ 121