REVENUES (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	$ 2,321,100		$ 6,458,900
Operating Revenues	2,271,425	$ 1,988,512	6,413,243	$ 5,856,458
Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,195,700		3,397,200
Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	847,100		2,312,900
Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	116,300		336,400
Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	2,159,100		6,046,500
Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	76,000		196,400
Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	61,500		185,900
Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	24,100		66,100
Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	400		(36,000)
Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(55,200)		(60,500)
Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	5,500		14,800
The Connecticut Light and Power Company
Disaggregation of Revenue [Line Items]
Operating Revenues	865,028	774,762	2,344,903	2,173,629
NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Operating Revenues	939,460	851,922	2,400,324	2,290,432
Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Operating Revenues	290,203	250,032	792,700	733,572
Operating Segments | Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	2,074,200		5,366,600
Operating Revenues	2,027,400	1,547,100	5,321,800	4,224,200
Operating Segments | Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,111,700		2,900,400
Operating Segments | Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	789,600		2,023,000
Operating Segments | Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	98,700		268,600
Operating Segments | Electric Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	2,000,000		5,192,000
Operating Segments | Electric Distribution | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Electric Distribution | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	48,800		141,400
Operating Segments | Electric Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	20,200		54,400
Operating Segments | Electric Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	5,200		(21,200)
Operating Segments | Electric Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(51,600)		(57,200)
Operating Segments | Electric Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	4,800		12,400
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	109,900		741,700
Operating Revenues	109,000	109,200	740,500	698,800
Operating Segments | Natural Gas Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	42,800		395,900
Operating Segments | Natural Gas Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	40,800		245,400
Operating Segments | Natural Gas Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	18,900		71,900
Operating Segments | Natural Gas Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	102,500		713,200
Operating Segments | Natural Gas Distribution | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Natural Gas Distribution | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	11,400		41,400
Operating Segments | Natural Gas Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(500)		(1,400)
Operating Segments | Natural Gas Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(3,500)		(11,500)
Operating Segments | Natural Gas Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(1,500)		(3,200)
Operating Segments | Natural Gas Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	600		2,000
Operating Segments | Electric Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	367,600		998,300
Operating Revenues	330,600	328,500	953,000	970,000
Operating Segments | Electric Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Electric Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Electric Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Electric Transmission | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Electric Transmission | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	364,500		988,900
Operating Segments | Electric Transmission | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Electric Transmission | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	3,100		9,400
Operating Segments | Electric Transmission | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Electric Transmission | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(37,000)		(45,300)
Operating Segments | Electric Transmission | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0		0
Operating Segments | Water Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	62,300		157,400
Operating Revenues	63,500	0	161,500	0
Operating Segments | Water Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	41,200		100,900
Operating Segments | Water Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17,900		47,900
Operating Segments | Water Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,300		3,400
Operating Segments | Water Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	60,400		152,200
Operating Segments | Water Distribution | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Water Distribution | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,300		3,100
Operating Segments | Water Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,900		5,400
Operating Segments | Water Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(1,300)		(3,300)
Operating Segments | Water Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,100		3,700
Operating Segments | Water Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	100		400
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	224,600		692,500
Operating Revenues	224,600		692,500
Operating Segments | Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Other | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Other | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Other | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Other | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	11,700		34,400
Operating Segments | Other | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Other | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	212,900		658,100
Operating Segments | Other | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Operating Segments | Other | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0		0
Operating Segments | Other | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0		0
Operating Segments | The Connecticut Light and Power Company
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,048,000		2,744,800
Operating Segments | The Connecticut Light and Power Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	539,600		1,410,000
Operating Segments | The Connecticut Light and Power Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	259,000		702,200
Operating Segments | The Connecticut Light and Power Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	39,400		111,800
Operating Segments | The Connecticut Light and Power Company | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	838,000		2,224,000
Operating Segments | The Connecticut Light and Power Company | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	179,100		469,800
Operating Segments | The Connecticut Light and Power Company | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	13,300		34,300
Operating Segments | The Connecticut Light and Power Company | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	9,300		25,000
Operating Segments | The Connecticut Light and Power Company | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	8,300		(8,300)
Operating Segments | NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,053,300		2,700,600
Operating Segments | NSTAR Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	414,800		1,071,600
Operating Segments | NSTAR Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	443,800		1,083,800
Operating Segments | NSTAR Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	38,000		96,900
Operating Segments | NSTAR Electric Company | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	896,600		2,252,300
Operating Segments | NSTAR Electric Company | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	128,300		367,700
Operating Segments | NSTAR Electric Company | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	18,100		56,400
Operating Segments | NSTAR Electric Company | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,300		27,900
Operating Segments | NSTAR Electric Company | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		(3,700)
Operating Segments | Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	341,300		923,000
Operating Segments | Public Service Company of New Hampshire | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	157,300		418,800
Operating Segments | Public Service Company of New Hampshire | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	87,300		238,400
Operating Segments | Public Service Company of New Hampshire | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	21,300		59,900
Operating Segments | Public Service Company of New Hampshire | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	265,900		717,100
Operating Segments | Public Service Company of New Hampshire | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	57,100		151,400
Operating Segments | Public Service Company of New Hampshire | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17,400		52,300
Operating Segments | Public Service Company of New Hampshire | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	4,000		11,400
Operating Segments | Public Service Company of New Hampshire | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(3,100)		(9,200)
Segment Reconciling Items [Member] | The Connecticut Light and Power Company | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(64,300)		(68,400)
Segment Reconciling Items [Member] | The Connecticut Light and Power Company | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	2,800		6,500
Segment Reconciling Items [Member] | NSTAR Electric Company | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(15,400)		(15,600)
Segment Reconciling Items [Member] | NSTAR Electric Company | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,800		5,100
Segment Reconciling Items [Member] | Public Service Company of New Hampshire | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(8,900)		(18,500)
Segment Reconciling Items [Member] | Public Service Company of New Hampshire | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	200		800
Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(517,500)		(1,497,600)
Operating Revenues	(483,700)	$ (220,500)	(1,456,100)	$ (714,000)
Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(1,200)		(3,400)
Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(2,600)		(7,500)
Eliminations | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(3,800)		(10,900)
Eliminations | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(300,200)		(826,900)
Eliminations | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Eliminations | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(213,500)		(659,800)
Eliminations | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0		0
Eliminations | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	33,800		41,500
Eliminations | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0		0
Consolidations Eliminations | The Connecticut Light and Power Company
Disaggregation of Revenue [Line Items]
Operating Revenues	(121,500)		(338,000)
Consolidations Eliminations | NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Operating Revenues	(100,200)		(289,800)
Consolidations Eliminations | Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Operating Revenues	$ (42,400)		$ (112,600)