REVENUES (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	$ 2,142,900	$ 1,947,200	$ 4,915,500	$ 4,241,700
Alternative Revenue Programs	(21,400)	4,900	30,300	79,400
Other Revenues	1,000	1,000	2,600	5,800
Operating Revenues	$ 2,122,538	$ 1,953,128	4,948,378	4,326,854
Operating Lease, Income, Comprehensive Income [Extensible Enumeration]	Operating Revenues	Operating Revenues
Lease revenue	$ 1,000	$ 1,100	2,700	2,200
CL&P
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 829,597	$ 817,421	1,816,871	1,717,124
Operating Lease, Income, Comprehensive Income [Extensible Enumeration]	Operating Revenues	Operating Revenues
Lease revenue	$ 200	$ 200	400	400
NSTAR Electric
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 687,375	$ 660,971	1,424,418	1,394,804
Operating Lease, Income, Comprehensive Income [Extensible Enumeration]	Operating Revenues	Operating Revenues
Lease revenue	$ 700	$ 700	1,800	1,400
PSNH
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 278,849	$ 255,224	572,284	531,592
Operating Lease, Income, Comprehensive Income [Extensible Enumeration]	Operating Revenues	Operating Revenues
Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	$ 1,092,300	$ 1,071,800	2,652,900	2,301,300
Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	698,500	608,400	1,478,500	1,363,700
Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	113,500	96,600	250,000	202,700
Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,904,300	1,776,800	4,381,400	3,867,700
Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	91,300	70,400	183,800	140,500
Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	113,900	71,500	290,100	176,500
Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	33,400	28,500	60,200	57,000
Operating Segments | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	958,200	940,700	2,060,900	1,916,300
Alternative Revenue Programs	(1,100)	(7,500)	7,800	37,100
Other Revenues	(200)	200	100	1,900
Operating Segments | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	824,200	764,100	1,662,700	1,583,600
Alternative Revenue Programs	(15,300)	3,600	(1,500)	23,000
Other Revenues	700	700	1,800	2,100
Operating Segments | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	319,600	284,300	653,300	592,900
Alternative Revenue Programs	2,300	7,900	5,300	12,600
Other Revenues	500	0	500	600
Operating Segments | Residential | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	440,800	469,300	986,600	959,200
Operating Segments | Residential | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	304,700	311,500	667,100	642,400
Operating Segments | Residential | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	136,600	135,200	294,400	278,100
Operating Segments | Commercial | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	215,700	199,300	429,900	425,600
Operating Segments | Commercial | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	296,100	261,000	564,400	562,900
Operating Segments | Commercial | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	82,800	66,100	160,600	145,300
Operating Segments | Industrial | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	28,400	33,300	65,200	67,300
Operating Segments | Industrial | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	29,100	24,800	53,600	51,600
Operating Segments | Industrial | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	24,500	19,600	46,200	38,600
Operating Segments | Total Retail Tariff Sales Revenues | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	684,900	701,900	1,481,700	1,452,100
Operating Segments | Total Retail Tariff Sales Revenues | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	629,900	597,300	1,285,100	1,256,900
Operating Segments | Total Retail Tariff Sales Revenues | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	243,900	220,900	501,200	462,000
Operating Segments | Wholesale Transmission Revenues | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	195,700	191,400	384,600	343,200
Operating Segments | Wholesale Transmission Revenues | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	160,100	142,500	307,200	277,300
Operating Segments | Wholesale Transmission Revenues | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	61,100	49,500	119,400	99,200
Operating Segments | Wholesale Market Sales Revenues | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	68,100	39,300	177,800	104,000
Operating Segments | Wholesale Market Sales Revenues | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	18,900	13,200	43,400	27,600
Operating Segments | Wholesale Market Sales Revenues | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	10,200	8,000	25,100	19,900
Operating Segments | Other Revenues from Contracts with Customers | CL&P
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	9,500	8,100	16,800	17,000
Operating Segments | Other Revenues from Contracts with Customers | NSTAR Electric
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	15,300	11,100	27,000	21,800
Operating Segments | Other Revenues from Contracts with Customers | PSNH
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	4,400	5,900	7,600	11,800
Other
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	330,000	283,800	673,000	578,500
Alternative Revenue Programs	0	0	0	0
Other Revenues	0	0	0	0
Operating Revenues	330,000	283,800	673,000	578,500
Other | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Other | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Other | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Other | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Other | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	20,300	19,200	39,500	36,600
Other | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Other | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	309,700	264,600	633,500	541,900
Eliminations
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(658,600)	(599,700)	(1,305,900)	(1,163,600)
Alternative Revenue Programs	(1,200)	9,400	1,000	(18,100)
Other Revenues	0	0	0	0
Operating Revenues	(659,800)	(590,300)	(1,304,900)	(1,181,700)
Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(1,300)	(1,300)	(2,700)	(2,300)
Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(4,400)	(3,400)	(8,000)	(6,600)
Eliminations | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(5,700)	(4,700)	(10,700)	(8,900)
Eliminations | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(345,900)	(332,200)	(666,900)	(615,800)
Eliminations | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Eliminations | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(307,000)	(262,800)	(628,300)	(538,900)
Eliminations | CL&P
Disaggregation of Revenue [Line Items]
Operating Revenues	(127,300)	(116,000)	(251,900)	(238,200)
Eliminations | NSTAR Electric
Disaggregation of Revenue [Line Items]
Operating Revenues	(122,200)	(107,400)	(238,600)	(213,900)
Eliminations | PSNH
Disaggregation of Revenue [Line Items]
Operating Revenues	(43,600)	(37,000)	(86,800)	(74,500)
Electric Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,680,500	1,601,300	3,556,800	3,364,300
Alternative Revenue Programs	(4,800)	14,200	18,200	53,100
Other Revenues	800	700	1,900	4,200
Operating Revenues	1,676,500	1,616,200	3,576,900	3,421,600
Electric Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	882,100	916,000	1,948,100	1,879,700
Electric Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	594,100	526,000	1,154,000	1,133,000
Electric Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	82,000	77,700	165,000	157,500
Electric Distribution | Operating Segments | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,558,200	1,519,700	3,267,100	3,170,200
Electric Distribution | Operating Segments | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Electric Distribution | Operating Segments | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	97,200	60,500	246,300	151,500
Electric Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	25,100	21,100	43,400	42,600
Natural Gas Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	315,300	216,500	1,073,700	632,000
Alternative Revenue Programs	(3,400)	(5,300)	18,600	26,900
Other Revenues	(100)	0	0	900
Operating Revenues	311,800	211,200	1,092,300	659,800
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	174,200	116,100	641,100	354,100
Natural Gas Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	89,600	67,800	297,400	202,400
Natural Gas Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	34,800	21,100	90,800	49,500
Natural Gas Distribution | Operating Segments | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	298,600	205,000	1,029,300	606,000
Natural Gas Distribution | Operating Segments | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Natural Gas Distribution | Operating Segments | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	15,700	10,100	42,000	23,200
Natural Gas Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,000	1,400	2,400	2,800
Electric Transmission | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	420,300	386,700	818,000	726,300
Alternative Revenue Programs	(9,300)	(10,200)	(6,600)	19,600
Other Revenues	200	200	500	400
Operating Revenues	411,200	376,700	811,900	746,300
Electric Transmission | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Electric Transmission | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Electric Transmission | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Electric Transmission | Operating Segments | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Electric Transmission | Operating Segments | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	416,900	383,400	811,200	719,700
Electric Transmission | Operating Segments | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Electric Transmission | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,400	3,300	6,800	6,600
Water Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	55,400	58,600	99,900	104,200
Alternative Revenue Programs	(2,700)	(3,200)	(900)	(2,100)
Other Revenues	100	100	200	300
Operating Revenues	52,800	55,500	99,200	102,400
Water Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	36,000	39,700	63,700	67,500
Water Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	16,100	15,900	29,800	30,600
Water Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,100	1,200	2,200	2,300
Water Distribution | Operating Segments | Total Retail Tariff Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	53,200	56,800	95,700	100,400
Water Distribution | Operating Segments | Wholesale Transmission Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0	0	0
Water Distribution | Operating Segments | Wholesale Market Sales Revenues
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,000	900	1,800	1,800
Water Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	$ 1,200	$ 900	$ 2,400	$ 2,000