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Variable Interest Entities (VIEs) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Variable Interest Entity [Line Items]        
Operating Revenues $ 2,133 $ 1,905 $ 4,725 $ 4,521
Operation and Maintenance 708 710 1,420 1,439
Long Island ServCo [Member]        
Variable Interest Entity [Line Items]        
Operating Revenues 112 101 224 199
Operation and Maintenance $ 112 $ 101 $ 224 $ 199