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REVENUES (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 $ 8,472,700    
Operating Revenues $ 2,034,900 $ 2,271,400 $ 1,853,900 $ 2,288,000 $ 1,895,600 $ 1,988,500 $ 1,762,800 $ 2,105,100 8,448,201 $ 7,751,952 $ 7,639,129
Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 4,439,800    
Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 3,028,300    
Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 442,300    
Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 7,910,400    
Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 264,000    
Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 241,100    
Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 81,500    
Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (24,300)    
Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (46,100)    
Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 21,600    
The Connecticut Light And Power Company                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues 751,300 865,000 694,900 785,000 713,700 774,800 666,600 732,300 3,096,174 2,887,359 2,805,955
NSTAR Electric Company                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues 712,600 939,500 690,700 770,100 690,200 851,900 704,700 733,800 3,112,926 2,980,629 3,041,588
Public Service Company Of New Hampshire                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues $ 254,900 $ 290,200 $ 235,100 $ 267,400 $ 248,000 $ 250,000 $ 230,400 $ 253,200 1,047,619 981,624 959,482
Operating Segments | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Electric Distribution                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 6,986,300    
Operating Revenues                 6,957,200    
Operating Segments | Electric Distribution | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 3,766,600    
Operating Segments | Electric Distribution | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 2,634,700    
Operating Segments | Electric Distribution | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 351,900    
Operating Segments | Electric Distribution | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 6,753,200    
Operating Segments | Electric Distribution | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Electric Distribution | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 179,500    
Operating Segments | Electric Distribution | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 65,900    
Operating Segments | Electric Distribution | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (12,300)    
Operating Segments | Electric Distribution | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (47,000)    
Operating Segments | Electric Distribution | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 17,900    
Operating Segments | Natural Gas Distribution                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 1,020,300    
Operating Revenues                 1,022,200 947,300 857,700
Operating Segments | Natural Gas Distribution | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 542,500    
Operating Segments | Natural Gas Distribution | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 334,800    
Operating Segments | Natural Gas Distribution | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 96,000    
Operating Segments | Natural Gas Distribution | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 973,300    
Operating Segments | Natural Gas Distribution | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Natural Gas Distribution | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 57,500    
Operating Segments | Natural Gas Distribution | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (2,200)    
Operating Segments | Natural Gas Distribution | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (8,300)    
Operating Segments | Natural Gas Distribution | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (1,200)    
Operating Segments | Natural Gas Distribution | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 3,100    
Operating Segments | Electric Transmission                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 1,321,500    
Operating Revenues                 1,286,300    
Operating Segments | Electric Transmission | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Electric Transmission | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Electric Transmission | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Electric Transmission | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Electric Transmission | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 1,308,900    
Operating Segments | Electric Transmission | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Electric Transmission | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 12,600    
Operating Segments | Electric Transmission | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (35,200)    
Operating Segments | Electric Transmission | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 0    
Operating Segments | Water Distribution                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 206,000    
Operating Revenues                 212,000    
Operating Segments | Water Distribution | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 130,700    
Operating Segments | Water Distribution | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 63,300    
Operating Segments | Water Distribution | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 4,400    
Operating Segments | Water Distribution | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 198,400    
Operating Segments | Water Distribution | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Water Distribution | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 4,100    
Operating Segments | Water Distribution | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 7,200    
Operating Segments | Water Distribution | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (3,700)    
Operating Segments | Water Distribution | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 5,400    
Operating Segments | Water Distribution | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 600    
Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 936,300    
Operating Revenues                 936,300    
Operating Segments | Other | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Other | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Other | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Other | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Other | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 47,300    
Operating Segments | Other | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Other | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 889,000    
Operating Segments | Other | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Other | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 0    
Operating Segments | Other | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 0    
Operating Segments | The Connecticut Light And Power Company                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 3,607,900    
Operating Segments | The Connecticut Light And Power Company | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 1,828,200    
Operating Segments | The Connecticut Light And Power Company | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 928,100    
Operating Segments | The Connecticut Light And Power Company | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 147,700    
Operating Segments | The Connecticut Light And Power Company | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 2,904,000    
Operating Segments | The Connecticut Light And Power Company | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 620,600    
Operating Segments | The Connecticut Light And Power Company | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 48,300    
Operating Segments | The Connecticut Light And Power Company | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 35,000    
Operating Segments | The Connecticut Light And Power Company | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | NSTAR Electric Company                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 3,491,100    
Operating Segments | NSTAR Electric Company | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 1,380,900    
Operating Segments | NSTAR Electric Company | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 1,391,500    
Operating Segments | NSTAR Electric Company | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 124,900    
Operating Segments | NSTAR Electric Company | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 2,897,300    
Operating Segments | NSTAR Electric Company | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 488,800    
Operating Segments | NSTAR Electric Company | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 76,100    
Operating Segments | NSTAR Electric Company | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 28,900    
Operating Segments | NSTAR Electric Company | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Operating Segments | Public Service Company Of New Hampshire                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 1,213,000    
Operating Segments | Public Service Company Of New Hampshire | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 557,500    
Operating Segments | Public Service Company Of New Hampshire | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 316,900    
Operating Segments | Public Service Company Of New Hampshire | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 79,300    
Operating Segments | Public Service Company Of New Hampshire | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 953,700    
Operating Segments | Public Service Company Of New Hampshire | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 199,500    
Operating Segments | Public Service Company Of New Hampshire | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 56,600    
Operating Segments | Public Service Company Of New Hampshire | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 15,500    
Operating Segments | Public Service Company Of New Hampshire | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (12,300)    
Segment Reconciling Items [Member] | The Connecticut Light And Power Company | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (65,900)    
Segment Reconciling Items [Member] | The Connecticut Light And Power Company | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 8,500    
Segment Reconciling Items [Member] | NSTAR Electric Company | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 900    
Segment Reconciling Items [Member] | NSTAR Electric Company | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 8,300    
Segment Reconciling Items [Member] | Public Service Company Of New Hampshire | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (17,300)    
Segment Reconciling Items [Member] | Public Service Company Of New Hampshire | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 1,100    
Eliminations                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (1,997,700)    
Operating Revenues                 (1,965,800) $ (986,800) $ (893,300)
Eliminations | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Eliminations | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (4,500)    
Eliminations | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (10,000)    
Eliminations | Total Retail Tariff Sale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (14,500)    
Eliminations | Transmission Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (1,092,200)    
Eliminations | Wholesale Revenue                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Eliminations | Other Revenue from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 (891,000)    
Eliminations | Reserve for Revenue Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenue from Contracts with Customers                 0    
Eliminations | Alternative Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 31,900    
Eliminations | Other Revenue                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 0    
Consolidations Eliminations | The Connecticut Light And Power Company                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (454,300)    
Consolidations Eliminations | NSTAR Electric Company                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (387,400)    
Consolidations Eliminations | Public Service Company Of New Hampshire                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 $ (149,200)