Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets:
Cash equivalents	$ 72	$ 255
Debt securities:
Domestic government	19	20
Corporate credit	12	13
Money market funds measured at NAV (2)	13	12
Non-qualified benefit plan trust: (2)
Money market funds	1	1
Equity securities - domestic	1	7
Debt securities—domestic government	8	1
Assets from price risk management activities: (1) (3)
Electricity	19	8
Natural gas	46	37
Total	13	12
Assets, Fair Value Disclosure	191	354
Liabilities from price risk management activities: (1) (3)
Electricity	134	146
Natural gas	4	5
Total	138	151
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	72	255
Debt securities:
Domestic government	12	9
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Money market funds	1	1
Equity securities - domestic	1	7
Debt securities—domestic government	8	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	94	273
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	11
Corporate credit	12	13
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	4
Natural gas	44	36
Total	77	64
Liabilities from price risk management activities: (1) (3)
Electricity	11	5
Natural gas	3	4
Total	14	9
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	5	4
Natural gas	2	1
Total	7	5
Liabilities from price risk management activities: (1) (3)
Electricity	123	141
Natural gas	1	1
Total	$ 124	$ 142