Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets:
Cash equivalents	$ 3	$ 26
Debt securities:
Domestic government	22	24
Corporate credit	10	9
Money market funds measured at NAV (2)	13	13
Non-qualified benefit plan trust: (2)
Money market funds	1	1
Equity securities - domestic	6	7
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	14	16
Natural gas	16	22
Total	13	13
Assets, Fair Value Disclosure	86	119
Liabilities from price risk management activities: (1) (3)
Electricity	143	119
Natural gas	18	12
Total	161	131
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	3	26
Debt securities:
Domestic government	7	8
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Money market funds	1	1
Equity securities - domestic	6	7
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	18	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	15	16
Corporate credit	10	9
Non-qualified benefit plan trust: (2)
Money market funds	0	0
Equity securities - domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	14	9
Natural gas	15	21
Total	54	55
Liabilities from price risk management activities: (1) (3)
Electricity	9	14
Natural gas	17	12
Total	26	26
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
Equity securities - domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	0	7
Natural gas	1	1
Total	1	8
Liabilities from price risk management activities: (1) (3)
Electricity	134	105
Natural gas	1	0
Total	$ 135	$ 105