Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Cash equivalents	$ 31	$ 30
Debt securities:
Domestic government	10	11
Corporate credit	5	6
Money market funds measured at NAV (2)	27	25
Non-qualified benefit plan trust: (2)
Money market funds	2	1
Eqauity securities - domestic	7	7
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	3	3
Natural gas	1	3
Total	27	25
Assets, Fair Value Disclosure	87	87
Liabilities from price risk management activities: (1) (3)
Electricity	120	135
Natural gas	67	75
Total	187	210
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	31	30
Debt securities:
Domestic government	3	4
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Money market funds	2	1
Eqauity securities - domestic	7	7
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	44	43
Liabilities from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash equivalents	0	0
Debt securities:
Domestic government	7	7
Corporate credit	5	6
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	1	3
Natural gas	1	3
Total	14	19
Liabilities from price risk management activities: (1) (3)
Electricity	4	5
Natural gas	52	66
Total	56	71
Fair Value, Inputs, Level 3 [Member]
Assets:
Cash equivalents	0	0
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	0
Natural gas	0	0
Total	2	0
Liabilities from price risk management activities: (1) (3)
Electricity	116	130
Natural gas	15	9
Total	$ 131	$ 139