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Variable Interest Entities (VIEs) (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Variable Interest Entity [Line Items]    
Operating Revenues $ 2,592 $ 2,616
Operation and Maintenance 712 729
Long Island ServCo [Member]    
Variable Interest Entity [Line Items]    
Operating Revenues 112 98
Operation and Maintenance $ 112 $ 98