Variable Interest Entities (VIEs) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Variable Interest Entity [Line Items]
Operating Revenues	$ 2,316	$ 2,016	$ 5,296	$ 4,834
Operation and Maintenance	750	725	1,506	1,479
Long Island ServCo [Member]
Variable Interest Entity [Line Items]
Operating Revenues	118	109	233	229
Operation and Maintenance	$ 118	$ 109	$ 233	$ 229