Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets:
Cash equivalents	$ 293	$ 26
Debt securities:
Domestic government	19	24
Corporate credit	15	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	28	16
Natural gas	28	22
Total	13	13
Assets, Fair Value Disclosure	404	119
Liabilities from price risk management activities: (1) (3)
Electricity	173	119
Natural gas	12	12
Total	185	131
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	293	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	308	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	15	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	28	9
Natural gas	25	21
Total	80	55
Liabilities from price risk management activities: (1) (3)
Electricity	19	14
Natural gas	12	12
Total	31	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	3	1
Total	3	8
Liabilities from price risk management activities: (1) (3)
Electricity	154	105
Natural gas	0	0
Total	$ 154	$ 105