Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019	Oct. 31, 2013
Storm Restoration Cost Recovery [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Minimum cost recovery period	12 months
Replenishment reserve for recovery of cost				$ 56
Multiple Hurricanes [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Storm restoration preparation costs	$ 10	$ 10	$ 10