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PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Public Utility Property Plant And Equipment [Line Items]    
Public Utilities Property Plant And Equipment Distribution $ 12,821,600 $ 12,495,200
Public Utilities Property Plant And Equipment Distribution Natural Gas 2,657,500 2,595,400
Public Utilities Property Plant And Equipment Transmission 7,260,600 6,930,700
Public Utilities Property Plant And Equipment Generation Or Processing 1,180,000 1,170,900
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total 23,919,700 23,192,200
Public Utilities Property Plant And Equipment Other Property Plant And Equipment 555,300 551,300
Public Utilities Property Plant And Equipment Plant In Service 24,475,000 23,743,500
Less Accumulated Depreciation [Abstract]    
Electric And Natural Gas Utility Accumulated Depreciation (6,041,500) (5,777,800)
Other Accumulated Depreciation (247,600) (231,800)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation (6,289,100) (6,009,600)
Property Plant And Equipment Net Excluding Construction Work In Progress 18,185,900 17,733,900
Public Utilities Property Plant And Equipment Construction Work In Progress 1,220,100 913,100
Property, Plant and Equipment, Net 19,406,025 18,647,041
The Connecticut Light And Power Company [Member]    
Public Utility Property Plant And Equipment [Line Items]    
Public Utilities Property Plant And Equipment Distribution 5,289,600 5,158,800
Public Utilities Property Plant And Equipment Transmission 3,480,500 3,274,000
Public Utilities Property Plant And Equipment Generation Or Processing 0 0
Public Utilities Property Plant And Equipment Plant In Service 8,770,100 8,432,800
Less Accumulated Depreciation [Abstract]    
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation (2,007,900) (1,928,000)
Property Plant And Equipment Net Excluding Construction Work In Progress 6,762,200 6,504,800
Public Utilities Property Plant And Equipment Construction Work In Progress 246,400 304,900
Property, Plant and Equipment, Net 7,008,603 6,809,664
NSTAR Electric Company [Member]    
Public Utility Property Plant And Equipment [Line Items]    
Public Utilities Property Plant And Equipment Distribution 5,001,900 4,895,500
Public Utilities Property Plant And Equipment Transmission 1,995,300 1,928,500
Public Utilities Property Plant And Equipment Generation Or Processing 0 0
Public Utilities Property Plant And Equipment Plant In Service 6,997,200 6,824,000
Less Accumulated Depreciation [Abstract]    
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation (1,857,900) (1,761,400)
Property Plant And Equipment Net Excluding Construction Work In Progress 5,139,300 5,062,600
Public Utilities Property Plant And Equipment Construction Work In Progress 405,800 272,800
Property, Plant and Equipment, Net 5,545,082 5,335,436
Public Service Company Of New Hampshire [Member]    
Public Utility Property Plant And Equipment [Line Items]    
Public Utilities Property Plant And Equipment Distribution 1,772,000 1,696,700
Public Utilities Property Plant And Equipment Transmission 839,900 789,700
Public Utilities Property Plant And Equipment Generation Or Processing 1,145,500 1,136,500
Public Utilities Property Plant And Equipment Plant In Service 3,757,400 3,622,900
Less Accumulated Depreciation [Abstract]    
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation (1,149,600) (1,090,000)
Property Plant And Equipment Net Excluding Construction Work In Progress 2,607,800 2,532,900
Public Utilities Property Plant And Equipment Construction Work In Progress 160,400 102,900
Property, Plant and Equipment, Net 2,768,206 2,635,844
Public Service Company Of New Hampshire [Member] | Generation Plant [Member]    
Less Accumulated Depreciation [Abstract]    
Electric And Natural Gas Utility Accumulated Depreciation (514,000)  
Western Massachusetts Electric Company [Member]    
Public Utility Property Plant And Equipment [Line Items]    
Public Utilities Property Plant And Equipment Distribution 798,100 784,200
Public Utilities Property Plant And Equipment Transmission 896,700 891,000
Public Utilities Property Plant And Equipment Generation Or Processing 34,500 34,400
Public Utilities Property Plant And Equipment Plant In Service 1,729,300 1,709,600
Less Accumulated Depreciation [Abstract]    
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation (302,600) (297,400)
Property Plant And Equipment Net Excluding Construction Work In Progress 1,426,700 1,412,200
Public Utilities Property Plant And Equipment Construction Work In Progress 105,000 49,100
Property, Plant and Equipment, Net $ 1,531,705 $ 1,461,321