Variable Interest Entities (VIEs) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Operating Revenues	$ 9,603	$ 10,076	$ 9,696
Operation and Maintenance	3,115	3,111	3,069
Long Island ServCo [Member]
Variable Interest Entity [Line Items]
Operating Revenues	520	490	458
Operation and Maintenance	$ 520	$ 490	$ 458