Fair Value of Financial Assets and Liabilities - Impact of Derivative Activities on Income (Details) MMBTU in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021 USD ($) MMBTU	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) MMBTU	Sep. 30, 2020 USD ($)	Dec. 31, 2020 MMBTU
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges		$ 0	$ 0	$ 0	$ 0
Natural Gas Commodity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	[1],[2]	2		2		0
Other Derivative Instruments | Natural Gas Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		$ 0	$ 0	$ (1)	$ (1)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		$ 3		$ 3

[1] Amounts are not reflective of net positions in the underlying commodities.

[2] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.