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REVENUES (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 $ 8,450,000 $ 8,472,700  
Alternative Revenue Programs                 53,200 (46,100)  
Other Revenues                 23,300 21,600  
Operating Revenues $ 2,050,400 $ 2,175,800 $ 1,884,500 $ 2,415,800 $ 2,034,900 $ 2,271,400 $ 1,853,900 $ 2,288,000 8,526,470 8,448,201 $ 7,751,952
Lease revenue                 4,400    
Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 4,411,100 4,439,800  
Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 2,992,000 3,028,300  
Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 421,600 442,300  
Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 7,824,700 7,910,400  
Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 269,400 264,000  
Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 275,200 241,100  
Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 76,000 81,500  
Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 4,700 (24,300)  
CL&P                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues 788,700 853,900 740,800 849,200 751,300 865,000 694,900 785,000 3,232,551 3,096,174 2,887,359
Lease revenue                 1,000    
NSTAR Electric                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues 686,400 878,700 681,900 797,600 712,600 939,500 690,700 770,100 3,044,642 3,112,926 2,980,629
Lease revenue                 2,700    
PSNH                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues $ 268,200 $ 280,400 $ 240,900 $ 276,400 $ 254,900 $ 290,200 $ 235,100 $ 267,400 1,065,936 1,047,619 981,624
Operating Segments | Electric Distribution                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 6,912,100 6,986,300  
Alternative Revenue Programs                 45,900 (47,000)  
Other Revenues                 18,500 17,900  
Operating Revenues                 6,976,500 6,957,200 5,542,900
Operating Segments | Electric Distribution | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 3,723,700 3,766,600  
Operating Segments | Electric Distribution | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 2,584,800 2,634,700  
Operating Segments | Electric Distribution | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 331,800 351,900  
Operating Segments | Electric Distribution | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 6,640,300 6,753,200  
Operating Segments | Electric Distribution | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Electric Distribution | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 215,700 179,500  
Operating Segments | Electric Distribution | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 54,800 65,900  
Operating Segments | Electric Distribution | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,300 (12,300)  
Operating Segments | Natural Gas Distribution                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,064,000 1,020,300  
Alternative Revenue Programs                 (4,900) (1,200)  
Other Revenues                 3,100 3,100  
Operating Revenues                 1,062,200 1,022,200 947,300
Operating Segments | Natural Gas Distribution | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 555,100 542,500  
Operating Segments | Natural Gas Distribution | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 347,600 334,800  
Operating Segments | Natural Gas Distribution | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 96,900 96,000  
Operating Segments | Natural Gas Distribution | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 999,600 973,300  
Operating Segments | Natural Gas Distribution | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Natural Gas Distribution | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 55,400 57,500  
Operating Segments | Natural Gas Distribution | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 2,800 (2,200)  
Operating Segments | Natural Gas Distribution | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 6,200 (8,300)  
Operating Segments | Electric Transmission                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,306,500 1,321,500  
Alternative Revenue Programs                 81,800 (35,200)  
Other Revenues                 700 0  
Operating Revenues                 1,389,000 1,286,300 1,301,700
Operating Segments | Electric Transmission | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Electric Transmission | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Electric Transmission | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Electric Transmission | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Electric Transmission | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,293,300 1,308,900  
Operating Segments | Electric Transmission | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Electric Transmission | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 13,200 12,600  
Operating Segments | Electric Transmission | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Water Distribution                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 209,000 206,000  
Alternative Revenue Programs                 4,600 5,400  
Other Revenues                 1,000 600  
Operating Revenues                 214,600 212,000  
Operating Segments | Water Distribution | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 132,300 130,700  
Operating Segments | Water Distribution | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 63,900 63,300  
Operating Segments | Water Distribution | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 4,500 4,400  
Operating Segments | Water Distribution | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 200,700 198,400  
Operating Segments | Water Distribution | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | Water Distribution | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 4,100 4,100  
Operating Segments | Water Distribution | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 7,000 7,200  
Operating Segments | Water Distribution | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 (2,800) (3,700)  
Operating Segments | CL&P                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 3,626,900 3,607,900  
Alternative Revenue Programs                 77,500 (65,900)  
Other Revenues                 10,300 8,500  
Operating Segments | CL&P | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,837,100 1,828,200  
Operating Segments | CL&P | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 922,900 928,100  
Operating Segments | CL&P | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 138,300 147,700  
Operating Segments | CL&P | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 2,898,300 2,904,000  
Operating Segments | CL&P | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 587,100 620,600  
Operating Segments | CL&P | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 105,100 48,300  
Operating Segments | CL&P | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 36,400 35,000  
Operating Segments | CL&P | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | NSTAR Electric                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 3,396,400 3,491,100  
Alternative Revenue Programs                 41,600 900  
Other Revenues                 7,000 8,300  
Operating Segments | NSTAR Electric | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,322,100 1,380,900  
Operating Segments | NSTAR Electric | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,349,400 1,391,500  
Operating Segments | NSTAR Electric | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 115,800 124,900  
Operating Segments | NSTAR Electric | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 2,787,300 2,897,300  
Operating Segments | NSTAR Electric | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 517,300 488,800  
Operating Segments | NSTAR Electric | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 73,100 76,100  
Operating Segments | NSTAR Electric | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 18,700 28,900  
Operating Segments | NSTAR Electric | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Operating Segments | PSNH                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,200,100 1,213,000  
Alternative Revenue Programs                 8,600 (17,300)  
Other Revenues                 1,900 1,100  
Operating Segments | PSNH | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 564,500 557,500  
Operating Segments | PSNH | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 314,600 316,900  
Operating Segments | PSNH | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 77,700 79,300  
Operating Segments | PSNH | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 956,800 953,700  
Operating Segments | PSNH | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 188,900 199,500  
Operating Segments | PSNH | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 37,500 56,600  
Operating Segments | PSNH | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 15,600 15,500  
Operating Segments | PSNH | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,300 (12,300)  
Other                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 1,028,500 936,300  
Alternative Revenue Programs                 0 0  
Other Revenues                 0 0  
Operating Revenues                 1,028,500 936,300 931,000
Other | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Other | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Other | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Other | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Other | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 61,300 47,300  
Other | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Other | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 967,200 889,000  
Other | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Eliminations                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 (2,070,100) (1,997,700)  
Alternative Revenue Programs                 (74,200) 31,900  
Other Revenues                 0 0  
Operating Revenues                 (2,144,300) (1,965,800) $ (986,800)
Eliminations | Residential                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Eliminations | Commercial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 (4,300) (4,500)  
Eliminations | Industrial                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 (11,600) (10,000)  
Eliminations | Total Retail Tariff Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 (15,900) (14,500)  
Eliminations | Wholesale Transmission Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 (1,085,200) (1,092,200)  
Eliminations | Wholesale Market Sales Revenues                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Eliminations | Other Revenues from Contracts with Customers                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 (969,000) (891,000)  
Eliminations | Reserve for Revenues Subject to Refund                      
Disaggregation of Revenue [Line Items]                      
Revenues from Contracts with Customers                 0 0  
Consolidations Eliminations | CL&P                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (482,100) (454,300)  
Consolidations Eliminations | NSTAR Electric                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 (400,400) (387,400)  
Consolidations Eliminations | PSNH                      
Disaggregation of Revenue [Line Items]                      
Operating Revenues                 $ (144,700) $ (149,200)