REVENUES (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	$ 2,294,400	$ 2,397,300
Alternative Revenue Programs	74,500	14,700
Other Revenues	4,800	3,800
Operating Revenues	2,373,726	2,415,792
Lease revenue	1,100	1,000
Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,229,600	1,319,200
Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	755,400	809,600
Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	106,000	111,400
Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,091,000	2,240,200
Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	70,000	67,600
Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	105,000	74,200
Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	26,400	17,600
Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,000	(2,300)
Other
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	294,600	258,100
Alternative Revenue Programs	0	0
Other Revenues	0	0
Operating Revenues	294,600	258,100
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 Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Other | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	17,300	13,500
Other | Wholesale Revenue
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	277,300	244,600
Other | Amortization/(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	(564,000)	(520,000)
Alternative Revenue Programs	(27,400)	(11,100)
Other Revenues	0	0
Operating Revenues	(591,400)	(531,100)
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	(1,000)	(1,100)
Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(3,200)	(2,700)
Eliminations | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(4,200)	(3,800)
Eliminations | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(283,600)	(270,800)
Eliminations | Wholesale Revenue
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	(276,200)	(245,400)
Eliminations | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,763,000	1,829,000
Alternative Revenue Programs	38,800	2,100
Other Revenues	3,500	2,800
Operating Revenues	1,805,300	1,833,900
Electric Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	963,800	1,033,300
Electric Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	607,000	652,600
Electric Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	79,800	82,100
Electric Distribution | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,650,600	1,768,000
Electric Distribution | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Distribution | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	91,000	51,500
Electric Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	19,100	12,600
Electric Distribution | Operating Segments | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,300	(3,100)
Natural Gas Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	415,500	457,800
Alternative Revenue Programs	32,200	10,400
Other Revenues	900	700
Operating Revenues	448,600	468,900
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	238,000	258,900
Natural Gas Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	134,600	143,800
Natural Gas Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	28,300	30,900
Natural Gas Distribution | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	400,900	433,600
Natural Gas Distribution | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Natural Gas Distribution | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	13,200	21,700
Natural Gas Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	900	900
Natural Gas Distribution | Operating Segments | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	500	1,600
Electric Transmission | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	339,700	328,100
Alternative Revenue Programs	29,800	12,400
Other Revenues	200	0
Operating Revenues	369,700	340,500
Electric Transmission | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Transmission | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Transmission | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Transmission | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Transmission | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	336,300	324,900
Electric Transmission | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Transmission | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,400	3,200
Electric Transmission | Operating Segments | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Water Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	45,600	44,300
Alternative Revenue Programs	1,100	900
Other Revenues	200	300
Operating Revenues	46,900	45,500
Water Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	27,800	27,000
Water Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	14,800	14,300
Water Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,100	1,100
Water Distribution | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	43,700	42,400
Water Distribution | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Water Distribution | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	800	1,000
Water Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,900	1,700
Water Distribution | Operating Segments | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(800)	(800)
CL&P
Disaggregation of Revenue [Line Items]
Operating Revenues	899,703	849,246
Lease revenue	200	200
CL&P | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	975,600	959,200
Alternative Revenue Programs	44,600	5,700
Other Revenues	1,700	1,000
CL&P | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	489,900	510,500
CL&P | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	226,400	236,700
CL&P | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	34,000	34,600
CL&P | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	750,300	781,800
CL&P | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	151,800	154,800
CL&P | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	64,700	13,700
CL&P | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	8,800	8,900
CL&P | Operating Segments | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
CL&P | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	(122,200)	(116,700)
NSTAR Electric
Disaggregation of Revenue [Line Items]
Operating Revenues	733,833	797,612
Lease revenue	700	600
NSTAR Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	819,600	887,300
Alternative Revenue Programs	19,300	7,300
Other Revenues	1,400	1,500
NSTAR Electric | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	331,000	371,000
NSTAR Electric | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	301,900	336,500
NSTAR Electric | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	26,800	28,800
NSTAR Electric | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	659,700	736,300
NSTAR Electric | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	134,800	122,600
NSTAR Electric | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	14,400	24,400
NSTAR Electric | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	10,700	4,000
NSTAR Electric | Operating Segments | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
NSTAR Electric | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	(106,500)	(98,500)
PSNH
Disaggregation of Revenue [Line Items]
Operating Revenues	276,368	276,435
PSNH | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	308,600	311,700
Alternative Revenue Programs	4,700	1,500
Other Revenues	600	300
PSNH | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	142,900	151,800
PSNH | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	79,100	79,800
PSNH | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	19,000	18,700
PSNH | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	241,000	250,300
PSNH | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	49,700	47,500
PSNH | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	11,900	13,400
PSNH | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,700	3,600
PSNH | Operating Segments | Amortization/(Reserve) for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,300	(3,100)
PSNH | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	$ (37,500)	$ (37,100)