Summary of Significant Accounting Policies - EEGH - Allowance for Doubtful Accounts (Details) - Eastern Energy Gas Holdings, LLC [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Doubtful Accounts [Line Items]
Allowance for doubtful accounts	$ 5	$ 2	$ 0	$ 0
Accounts Receivable, Credit Loss Expense (Reversal)	4	2	0
Accounts Receivable, Allowance for Credit Loss, Writeoff	(1)	0	0
Allowance for doubtful accounts	$ 5	$ 2	$ 0