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Variable Interest Entities (VIEs) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Variable Interest Entity [Line Items]        
Operating Revenues $ 2,263 $ 2,450 $ 6,988 $ 6,971
Operation and Maintenance 680 776 2,100 2,215
Long Island ServCo [Member]        
Variable Interest Entity [Line Items]        
Operating Revenues 114 116 338 315
Operation and Maintenance $ 114 $ 116 $ 338 $ 315