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Variable Interest Entities (VIEs) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Variable Interest Entity [Line Items]        
Operating Revenues $ 2,394 $ 2,254 $ 7,228 $ 6,987
Operation and Maintenance 742 693 2,221 2,128
Long Island ServCo [Member]        
Variable Interest Entity [Line Items]        
Operating Revenues 126 114 355 338
Operation and Maintenance $ 126 $ 114 $ 355 $ 338