Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets:
Cash equivalents	$ 42	$ 255
Debt securities:
Domestic government	18	20
Corporate credit	13	13
Money market funds measured at NAV (2)	12	12
Non-qualified benefit plan trust: (2)
Money market funds	2	1
Equity securities - domestic	7	7
Debt securities—domestic government	0	1
Assets from price risk management activities: (1) (3)
Electricity	43	8
Natural gas	229	37
Total	12	12
Assets, Fair Value Disclosure	366	354
Liabilities from price risk management activities: (1) (3)
Electricity	126	146
Natural gas	2	5
Total	128	151
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	42	255
Debt securities:
Domestic government	10	9
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Money market funds	2	1
Equity securities - domestic	7	7
Debt securities—domestic government	0	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	61	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	8	11
Corporate credit	13	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	31	4
Natural gas	205	36
Total	257	64
Liabilities from price risk management activities: (1) (3)
Electricity	36	5
Natural gas	2	4
Total	38	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	12	4
Natural gas	24	1
Total	36	5
Liabilities from price risk management activities: (1) (3)
Electricity	90	141
Natural gas	0	1
Total	$ 90	$ 142