REVENUES (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	$ 2,397,300	$ 2,277,100
Operating Revenues	2,415,792	2,287,962
Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,319,200	1,269,700
Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	809,600	759,900
Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	111,400	109,500
Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	2,240,200	2,139,100
Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	67,600	65,000
Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	74,200	77,100
Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	17,600	21,900
Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(2,300)	(26,000)
Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	14,700	6,600
Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	3,800	4,300
Lease revenue	1,000
Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers
Operating Revenues
Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers
Other
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	258,100	230,900
Operating Revenues	258,100	230,900
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	13,500	10,100
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	244,600	220,800
Other | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Other | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0	0
Other | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	0	0
Eliminations
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(520,000)	(482,600)
Operating Revenues	(531,100)	(472,000)
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	(1,100)	0
Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(2,700)	(2,500)
Eliminations | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(3,800)	(2,500)
Eliminations | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(270,800)	(258,700)
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	(245,400)	(221,400)
Eliminations | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Eliminations | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(11,100)	10,600
Eliminations | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	0	0
Electric Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,829,000	1,742,500
Operating Revenues	1,833,900	1,754,600
Electric Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,033,300	994,400
Electric Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	652,600	611,400
Electric Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	82,100	81,500
Electric Distribution | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,768,000	1,687,300
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	51,500	58,500
Electric Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	12,600	16,000
Electric Distribution | Operating Segments | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(3,100)	(19,300)
Electric Distribution | Operating Segments | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	2,100	8,700
Electric Distribution | Operating Segments | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	2,800	3,400
Natural Gas Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	457,800	425,800
Operating Revenues	468,900	424,900
Natural Gas Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	258,900	248,900
Natural Gas Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	143,800	134,700
Natural Gas Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	30,900	29,500
Natural Gas Distribution | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	433,600	413,100
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	21,700	17,800
Natural Gas Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	900	(600)
Natural Gas Distribution | Operating Segments | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,600	(4,500)
Natural Gas Distribution | Operating Segments | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	10,400	(1,700)
Natural Gas Distribution | Operating Segments | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	700	800
Electric Transmission | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	328,100	316,700
Operating Revenues	340,500	305,000
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	324,900	313,600
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	3,200	3,100
Electric Transmission | Operating Segments | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	0
Electric Transmission | Operating Segments | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	12,400	(11,700)
Electric Transmission | Operating Segments | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	0	0
Water Distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	44,300	43,800
Operating Revenues	45,500	44,600
Water Distribution | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	27,000	26,400
Water Distribution | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	14,300	13,800
Water Distribution | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,100	1,000
Water Distribution | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	42,400	41,200
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	1,000	800
Water Distribution | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	1,700	4,000
Water Distribution | Operating Segments | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(800)	(2,200)
Water Distribution | Operating Segments | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	900	700
Water Distribution | Operating Segments | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	300	100
CL&P
Disaggregation of Revenue [Line Items]
Operating Revenues	849,246	784,983
CL&P | Other Revenues
Disaggregation of Revenue [Line Items]
Lease revenue	200
CL&P | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	959,200	897,800
CL&P | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	510,500	483,400
CL&P | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	236,700	222,500
CL&P | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	34,600	35,800
CL&P | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	781,800	741,700
CL&P | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	154,800	150,800
CL&P | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	13,700	10,300
CL&P | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	8,900	7,500
CL&P | Operating Segments | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	(12,500)
CL&P | Segment Reconciling Items | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	5,700	(5,100)
CL&P | Segment Reconciling Items | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	1,000	1,300
CL&P | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	(116,700)	(109,000)
NSTAR Electric
Disaggregation of Revenue [Line Items]
Operating Revenues	797,612	770,127
NSTAR Electric | Other Revenues
Disaggregation of Revenue [Line Items]
Lease revenue	600
NSTAR Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	887,300	854,800
NSTAR Electric | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	371,000	364,200
NSTAR Electric | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	336,500	314,400
NSTAR Electric | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	28,800	28,100
NSTAR Electric | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	736,300	706,700
NSTAR Electric | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	122,600	118,600
NSTAR Electric | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	24,400	24,900
NSTAR Electric | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	4,000	8,300
NSTAR Electric | Operating Segments | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	0	(3,700)
NSTAR Electric | Segment Reconciling Items | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	7,300	6,700
NSTAR Electric | Segment Reconciling Items | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	1,500	1,700
NSTAR Electric | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	(98,500)	(93,100)
PSNH
Disaggregation of Revenue [Line Items]
Operating Revenues	276,435	267,350
PSNH | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	311,700	308,000
PSNH | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	151,800	146,800
PSNH | Operating Segments | Commercial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	79,800	74,900
PSNH | Operating Segments | Industrial
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	18,700	17,600
PSNH | Operating Segments | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	250,300	239,300
PSNH | Operating Segments | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	47,500	44,200
PSNH | Operating Segments | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	13,400	24,100
PSNH | Operating Segments | Other Revenues from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	3,600	3,500
PSNH | Operating Segments | Reserve for Revenues Subject to Refund
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers	(3,100)	(3,100)
PSNH | Segment Reconciling Items | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,500	(4,600)
PSNH | Segment Reconciling Items | Other Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues	300	400
PSNH | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	$ (37,100)	$ (36,400)