Variable Interest Entities (VIEs) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Operating Revenues	$ 2,478	$ 2,302	$ 2,316	$ 2,980	$ 2,468	$ 2,394	$ 2,016	$ 2,818	$ 10,076	$ 9,696	$ 9,094
Operation and Maintenance									3,111	3,069	2,901
Long Island ServCo [Member]
Variable Interest Entity [Line Items]
Operating Revenues									$ 490	458	438
Operation and Maintenance										$ 458	$ 438