Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets:
Cash equivalents	$ 228	$ 26
Debt securities:
Domestic government	20	24
Corporate credit	14	9
Money market funds measured at NAV (2)	13	13
Non-qualified benefit plan trust: (2)
Money market funds	1	1
Equity securities - domestic	7	7
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	11	16
Natural gas	70	22
Total	13	13
Assets, Fair Value Disclosure	365	119
Liabilities from price risk management activities: (1) (3)
Electricity	149	119
Natural gas	5	12
Total	154	131
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	228	26
Debt securities:
Domestic government	8	8
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Money market funds	1	1
Equity 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	245	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	12	16
Corporate credit	14	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	9	9
Natural gas	60	21
Total	95	55
Liabilities from price risk management activities: (1) (3)
Electricity	3	14
Natural gas	5	12
Total	8	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	2	7
Natural gas	10	1
Total	12	8
Liabilities from price risk management activities: (1) (3)
Electricity	146	105
Natural gas	0	0
Total	$ 146	$ 105