REVENUES - Schedule of Revenue from Contract with Customer (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	$ 2,277,100
Operating Revenues	2,287,962	$ 2,105,135
Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,269,700
Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	759,900
Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	109,500
Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	2,139,100
Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	65,000
Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	77,100
Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	21,900
Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	26,000
Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	6,600
Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	4,300
Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(482,600)
Operating Revenues	(472,000)
Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(2,500)
Eliminations | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(2,500)
Eliminations | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(258,700)
Eliminations | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Eliminations | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(221,400)
Eliminations | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Eliminations | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	10,600
Eliminations | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0
The Connecticut Light And Power Company
Disaggregation of Revenue [Line Items]
Operating Revenues	784,983	732,310
The Connecticut Light And Power Company | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	(109,000)
NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Operating Revenues	770,127	733,808
NSTAR Electric Company | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	(93,100)
Public Service Company Of New Hampshire
Disaggregation of Revenue [Line Items]
Operating Revenues	267,350	253,157
Public Service Company Of New Hampshire | Eliminations
Disaggregation of Revenue [Line Items]
Operating Revenues	(36,400)
Operating Segments | Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,742,500
Operating Revenues	1,754,600	1,401,100
Operating Segments | Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	994,400
Operating Segments | Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	611,400
Operating Segments | Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	81,500
Operating Segments | Electric Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,687,300
Operating Segments | Electric Distribution | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Electric Distribution | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	58,500
Operating Segments | Electric Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	16,000
Operating Segments | Electric Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	19,300
Operating Segments | Electric Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	8,700
Operating Segments | Electric Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	3,400
Operating Segments | Natural Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	425,800
Operating Revenues	424,900	403,600
Operating Segments | Natural Gas Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	248,900
Operating Segments | Natural Gas Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	134,700
Operating Segments | Natural Gas Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	29,500
Operating Segments | Natural Gas Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	413,100
Operating Segments | Natural Gas Distribution | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Natural Gas Distribution | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17,800
Operating Segments | Natural Gas Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(600)
Operating Segments | Natural Gas Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	4,500
Operating Segments | Natural Gas Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(1,700)
Operating Segments | Natural Gas Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	800
Operating Segments | Electric Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	316,700
Operating Revenues	305,000	316,900
Operating Segments | Electric Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Electric Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Electric Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Electric Transmission | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Electric Transmission | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	313,600
Operating Segments | Electric Transmission | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Electric Transmission | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	3,100
Operating Segments | Electric Transmission | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Electric Transmission | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(11,700)
Operating Segments | Electric Transmission | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0
Operating Segments | Water Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	43,800
Operating Revenues	44,600	$ 0
Operating Segments | Water Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	26,400
Operating Segments | Water Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	13,800
Operating Segments | Water Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,000
Operating Segments | Water Distribution | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	41,200
Operating Segments | Water Distribution | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Water Distribution | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	800
Operating Segments | Water Distribution | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	4,000
Operating Segments | Water Distribution | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	2,200
Operating Segments | Water Distribution | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	700
Operating Segments | Water Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	100
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	230,900
Operating Revenues	230,900
Operating Segments | Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Other | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Other | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Other | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Other | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,100
Operating Segments | Other | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Other | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	220,800
Operating Segments | Other | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Operating Segments | Other | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0
Operating Segments | Other | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	0
Operating Segments | The Connecticut Light And Power Company
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	897,800
Operating Revenues	785,000
Operating Segments | The Connecticut Light And Power Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	483,400
Operating Segments | The Connecticut Light And Power Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	222,500
Operating Segments | The Connecticut Light And Power Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	35,800
Operating Segments | The Connecticut Light And Power Company | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	741,700
Operating Segments | The Connecticut Light And Power Company | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	150,800
Operating Segments | The Connecticut Light And Power Company | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,300
Operating Segments | The Connecticut Light And Power Company | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	7,500
Operating Segments | The Connecticut Light And Power Company | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	12,500
Operating Segments | The Connecticut Light And Power Company | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(5,100)
Operating Segments | The Connecticut Light And Power Company | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,300
Operating Segments | NSTAR Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	854,800
Operating Revenues	770,100
Operating Segments | NSTAR Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	364,200
Operating Segments | NSTAR Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	314,400
Operating Segments | NSTAR Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	28,100
Operating Segments | NSTAR Electric Company | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	706,700
Operating Segments | NSTAR Electric Company | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	118,600
Operating Segments | NSTAR Electric Company | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	24,900
Operating Segments | NSTAR Electric Company | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	8,300
Operating Segments | NSTAR Electric Company | Reserve for Revenue Subject to Refund
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	3,700
Operating Segments | NSTAR Electric Company | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	6,700
Operating Segments | NSTAR Electric Company | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	1,700
Operating Segments | Public Service Company Of New Hampshire
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	308,000
Operating Revenues	267,400
Operating Segments | Public Service Company Of New Hampshire | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	146,800
Operating Segments | Public Service Company Of New Hampshire | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	74,900
Operating Segments | Public Service Company Of New Hampshire | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17,600
Operating Segments | Public Service Company Of New Hampshire | Total Retail Tariff Sale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	239,300
Operating Segments | Public Service Company Of New Hampshire | Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	44,200
Operating Segments | Public Service Company Of New Hampshire | Wholesale Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	24,100
Operating Segments | Public Service Company Of New Hampshire | Other Revenue from Contracts with Customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	3,500
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
Operating Segments | Public Service Company Of New Hampshire | Alternative Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	(4,600)
Operating Segments | Public Service Company Of New Hampshire | Other Revenue
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 400