REVENUES - Schedule of Revenue from Contract with Customer (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	$ 1,860,600		$ 4,138,900
Operating Revenues	1,853,856	$ 1,762,811	4,141,818	$ 3,867,946
Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	930,800		2,201,300
Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	705,700		1,466,000
Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	110,600		220,100
Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,747,100		3,887,400
Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	55,400		120,500
Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	47,200		124,300
Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	22,200		43,200
Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	11,300		36,500
Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(11,900)		(5,400)
Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	5,200		8,300
The Connecticut Light and Power Company
Disaggregation of Revenue [Line Items]
Operating Revenues	694,892	666,558	1,479,875	1,398,867
NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Operating Revenues	690,737	704,702	1,460,865	1,438,510
Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Operating Revenues	235,146	230,383	502,497	483,541
Operating Segments | Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,550,100		3,293,500
Operating Revenues	1,539,900	1,275,900	3,294,500	2,677,000
Operating Segments | Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	794,400		1,788,700
Operating Segments | Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	622,000		1,233,400
Operating Segments | Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	88,400		169,900
Operating Segments | Electric Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,504,800		3,192,000
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	34,200		92,700
Operating Segments | Electric Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	18,400		35,300
Operating Segments | Electric Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	7,300		26,500
Operating Segments | Electric Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(14,400)		(5,700)
Operating Segments | Electric Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	4,200		6,700
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	205,600		631,700
Operating Revenues	206,500	186,000	631,400	589,600
Operating Segments | Natural Gas Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	104,100		352,900
Operating Segments | Natural Gas Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	70,000		204,700
Operating Segments | Natural Gas Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	23,500		53,100
Operating Segments | Natural Gas Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	197,600		610,700
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	12,100		29,900
Operating Segments | Natural Gas Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(600)		(900)
Operating Segments | Natural Gas Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	3,500		8,000
Operating Segments | Natural Gas Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	100		(1,700)
Operating Segments | Natural Gas Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	800		1,400
Operating Segments | Electric Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	314,000		630,700
Operating Revenues	317,400	324,600	622,400	641,500
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	310,800		624,500
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	3,200		6,200
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	3,400		(8,300)
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	51,400		95,200
Operating Revenues	53,500	0	98,000	0
Operating Segments | Water Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	32,300		59,700
Operating Segments | Water Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	15,700		30,000
Operating Segments | Water Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,100		2,100
Operating Segments | Water Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	49,100		91,800
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	900		1,700
Operating Segments | Water Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,900		3,700
Operating Segments | Water Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	500		2,000
Operating Segments | Water Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,900		2,600
Operating Segments | Water Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	200		200
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	237,000		467,900
Operating Revenues	237,000		467,900
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	12,600		22,700
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	224,400		445,200
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	798,900		1,697,100
Operating Segments | The Connecticut Light and Power Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	387,100		870,400
Operating Segments | The Connecticut Light and Power Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	220,700		443,200
Operating Segments | The Connecticut Light and Power Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	36,500		72,300
Operating Segments | The Connecticut Light and Power Company | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	644,300		1,385,900
Operating Segments | The Connecticut Light and Power Company | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	139,900		290,600
Operating Segments | The Connecticut Light and Power Company | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,600		21,000
Operating Segments | The Connecticut Light and Power Company | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	8,300		16,200
Operating Segments | The Connecticut Light and Power Company | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	4,200		(16,600)
Operating Segments | NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	792,500		1,648,000
Operating Segments | NSTAR Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	292,500		656,700
Operating Segments | NSTAR Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	325,500		640,000
Operating Segments | NSTAR Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	30,900		59,000
Operating Segments | NSTAR Electric Company | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	648,900		1,355,700
Operating Segments | NSTAR Electric Company | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	120,900		239,500
Operating Segments | NSTAR Electric Company | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	13,400		38,200
Operating Segments | NSTAR Electric Company | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	9,300		18,300
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	273,800		581,800
Operating Segments | Public Service Company of New Hampshire | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	114,800		261,600
Operating Segments | Public Service Company of New Hampshire | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	76,200		151,100
Operating Segments | Public Service Company of New Hampshire | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	21,000		38,600
Operating Segments | Public Service Company of New Hampshire | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	212,000		451,300
Operating Segments | Public Service Company of New Hampshire | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	50,100		94,300
Operating Segments | Public Service Company of New Hampshire | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,900		34,900
Operating Segments | Public Service Company of New Hampshire | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	3,900		7,500
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		(6,200)
Segment Reconciling Items [Member] | The Connecticut Light and Power Company | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,000		(4,100)
Segment Reconciling Items [Member] | The Connecticut Light and Power Company | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	2,400		3,300
Segment Reconciling Items [Member] | NSTAR Electric Company | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(6,900)		(200)
Segment Reconciling Items [Member] | NSTAR Electric Company | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,600		2,700
Segment Reconciling Items [Member] | Public Service Company of New Hampshire | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(5,000)		(9,600)
Segment Reconciling Items [Member] | Public Service Company of New Hampshire | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	200		600
Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(497,500)		(980,100)
Operating Revenues	(500,400)	$ (240,700)	(972,400)	$ (493,600)
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	(2,000)		(2,100)
Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(2,400)		(5,000)
Eliminations | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(4,400)		(7,100)
Eliminations | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(268,000)		(526,700)
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	(225,100)		(446,300)
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	(2,900)		7,700
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	(107,400)		(216,400)
Consolidations Eliminations | NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Operating Revenues	(96,500)		(189,600)
Consolidations Eliminations | Public Service Company of New Hampshire
Disaggregation of Revenue [Line Items]
Operating Revenues	$ (33,900)		$ (70,300)