Fair Value of Financial Assets and Liabilities - Impact of Derivative Activities on Income (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	$ 0	$ 0	$ 0	$ 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	(1,000,000)	$ (1,000,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	$ 0		$ 1,000,000