Variable Interest Entities (VIEs) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Variable Interest Entity [Line Items]
Operating Revenues	$ 2,016	$ 2,142	$ 4,834	$ 4,733
Operation and Maintenance	725	718	1,479	1,435
Long Island ServCo [Member]
Variable Interest Entity [Line Items]
Operating Revenues	109	112	229	224
Operation and Maintenance	$ 109	$ 112	$ 229	$ 224