REVENUES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	$ 8,766,500	$ 8,450,000	$ 8,472,700
Alternative Revenue Programs	130,000	53,200	(46,100)
Other Revenues	7,900	23,300	21,600
Operating Revenues	8,904,430	8,526,470	8,448,201
Lease revenue	4,300	4,400
Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	4,741,500	4,411,100	4,439,800
Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,772,900	2,992,000	3,028,300
Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	425,600	421,600	442,300
Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	7,940,000	7,824,700	7,910,400
Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	340,900	269,400	264,000
Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	374,100	275,200	241,100
Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	108,600	76,000	81,500
Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,900	4,700	(24,300)
The Connecticut Light and Power Company
Disaggregation of Revenue [Line Items]
Operating Revenues	3,547,527	3,232,551	3,096,174
Lease revenue	800	1,000
NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Operating Revenues	2,941,148	3,044,642	3,112,926
Lease revenue	2,700	2,700
Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Operating Revenues	1,079,095	1,065,936	1,047,619
Operating Segments | Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	7,038,600	6,912,100	6,986,300
Alternative Revenue Programs	88,100	45,900	(47,000)
Other Revenues	5,600	18,500	17,900
Operating Revenues	7,132,300	6,976,500	6,957,200
Operating Segments | Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,951,500	3,723,700	3,766,600
Operating Segments | Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,353,400	2,584,800	2,634,700
Operating Segments | Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	327,100	331,800	351,900
Operating Segments | Electric Distribution | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	6,632,000	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	0
Operating Segments | Electric Distribution | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	327,300	215,700	179,500
Operating Segments | Electric Distribution | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	74,700	54,800	65,900
Operating Segments | Electric Distribution | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	4,600	1,300	(12,300)
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,162,900	1,064,000	1,020,300
Alternative Revenue Programs	44,700	(4,900)	(1,200)
Other Revenues	1,100	3,100	3,100
Operating Revenues	1,208,700	1,062,200	1,022,200
Operating Segments | Natural Gas Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	644,900	555,100	542,500
Operating Segments | Natural Gas Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	361,900	347,600	334,800
Operating Segments | Natural Gas Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	107,400	96,900	96,000
Operating Segments | Natural Gas Distribution | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,114,200	999,600	973,300
Operating Segments | Natural Gas Distribution | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Natural Gas Distribution | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	43,000	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	3,600	2,800	(2,200)
Operating Segments | Natural Gas Distribution | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,100	6,200	(8,300)
Operating Segments | Electric Transmission
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,570,600	1,306,500	1,321,500
Alternative Revenue Programs	(35,200)	81,800	(35,200)
Other Revenues	700	700	0
Operating Revenues	1,536,100	1,389,000	1,286,300
Operating Segments | Electric Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Electric Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Electric Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Electric Transmission | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Electric Transmission | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,557,300	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	0
Operating Segments | Electric Transmission | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	13,300	13,200	12,600
Operating Segments | Electric Transmission | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Water Distribution
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	219,600	209,000	206,000
Alternative Revenue Programs	(4,700)	4,600	5,400
Other Revenues	500	1,000	600
Operating Revenues	215,400	214,600	212,000
Operating Segments | Water Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	145,100	132,300	130,700
Operating Segments | Water Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	62,400	63,900	63,300
Operating Segments | Water Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	4,800	4,500	4,400
Operating Segments | Water Distribution | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	212,300	200,700	198,400
Operating Segments | Water Distribution | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Water Distribution | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,800	4,100	4,100
Operating Segments | Water Distribution | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	7,300	7,000	7,200
Operating Segments | Water Distribution | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(3,800)	(2,800)	(3,700)
Operating Segments | The Connecticut Light and Power Company
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	4,044,700	3,626,900	3,607,900
Alternative Revenue Programs	(4,200)	77,500	(65,900)
Other Revenues	2,200	10,300	8,500
Operating Segments | The Connecticut Light and Power Company | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,011,100	1,837,100	1,828,200
Operating Segments | The Connecticut Light and Power Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	878,300	922,900	928,100
Operating Segments | The Connecticut Light and Power Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	137,500	138,300	147,700
Operating Segments | The Connecticut Light and Power Company | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,026,900	2,898,300	2,904,000
Operating Segments | The Connecticut Light and Power Company | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	754,800	587,100	620,600
Operating Segments | The Connecticut Light and Power Company | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	230,100	105,100	48,300
Operating Segments | The Connecticut Light and Power Company | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	32,900	36,400	35,000
Operating Segments | The Connecticut Light and Power Company | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,327,500	3,396,400	3,491,100
Alternative Revenue Programs	54,500	41,600	900
Other Revenues	3,500	7,000	8,300
Operating Segments | NSTAR Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,365,800	1,322,100	1,380,900
Operating Segments | NSTAR Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,176,800	1,349,400	1,391,500
Operating Segments | NSTAR Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	106,400	115,800	124,900
Operating Segments | NSTAR Electric Company | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,649,000	2,787,300	2,897,300
Operating Segments | NSTAR Electric Company | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	576,500	517,300	488,800
Operating Segments | NSTAR Electric Company | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	58,400	73,100	76,100
Operating Segments | NSTAR Electric Company | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	43,600	18,700	28,900
Operating Segments | NSTAR Electric Company | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Operating Segments | Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,241,300	1,200,100	1,213,000
Alternative Revenue Programs	2,600	8,600	(17,300)
Other Revenues	600	1,900	1,100
Operating Segments | Public Service Company of New Hampshire | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	574,600	564,500	557,500
Operating Segments | Public Service Company of New Hampshire | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	299,900	314,600	316,900
Operating Segments | Public Service Company of New Hampshire | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	83,200	77,700	79,300
Operating Segments | Public Service Company of New Hampshire | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	957,700	956,800	953,700
Operating Segments | Public Service Company of New Hampshire | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	226,000	188,900	199,500
Operating Segments | Public Service Company of New Hampshire | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	38,800	37,500	56,600
Operating Segments | Public Service Company of New Hampshire | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	14,200	15,600	15,500
Operating Segments | Public Service Company of New Hampshire | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	4,600	1,300	(12,300)
Other
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,235,900	1,028,500	936,300
Alternative Revenue Programs	0	0	0
Other Revenues	0	0	0
Operating Revenues	1,235,900	1,028,500	936,300
Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Other | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Other | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Other | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Other | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	74,200	61,300	47,300
Other | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Other | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,161,700	967,200	889,000
Other | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Eliminations
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(2,461,100)	(2,070,100)	(1,997,700)
Alternative Revenue Programs	37,100	(74,200)	31,900
Other Revenues	0	0	0
Operating Revenues	(2,424,000)	(2,144,300)	(1,965,800)
Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(4,800)	(4,300)	(4,500)
Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(13,700)	(11,600)	(10,000)
Eliminations | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(18,500)	(15,900)	(14,500)
Eliminations | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(1,290,600)	(1,085,200)	(1,092,200)
Eliminations | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Eliminations | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(1,152,000)	(969,000)	(891,000)
Eliminations | Amortization of/(Reserve for) Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0
Consolidations Eliminations | The Connecticut Light and Power Company
Disaggregation of Revenue [Line Items]
Operating Revenues	(495,200)	(482,100)	(454,300)
Consolidations Eliminations | NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Operating Revenues	(444,400)	(400,400)	(387,400)
Consolidations Eliminations | Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Operating Revenues	$ (165,400)	$ (144,700)	$ (149,200)