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Variable Interest Entities (VIEs) (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Variable Interest Entity [Line Items]    
Operating Revenues $ 2,818 $ 2,591
Operation and Maintenance 754 717
Long Island ServCo [Member]    
Variable Interest Entity [Line Items]    
Operating Revenues 120 112
Operation and Maintenance $ 120 $ 112