REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 17, 2018	May 08, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 5,153,300		$ 5,153,300				$ 5,239,300
Other Income, Net	16,718	$ 28,536	100,656	$ 79,178
Amount of regulatory costs not yet approved	104,600		104,600				105,800
Regulatory liabilities	4,051,500		4,051,500				3,765,400
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets	953,500		953,500				940,800
Amount of recovery approved					$ 49,000
Other Income, Net	16,095	2,664	24,253	7,367
Amount of regulatory costs not yet approved	3,900		3,900				27,200
Regulatory liabilities	473,500		473,500				450,700
The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Regulatory assets	1,673,800		1,673,800				1,645,200
Other Income, Net	7,098	7,913	20,722	15,402
Amount of regulatory costs not yet approved	27,600		27,600				18,200
Regulatory liabilities	1,257,800		1,257,800				1,151,100
NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory assets	1,420,300		1,420,300				1,524,500
Other Income, Net	13,697	$ 9,165	40,567	$ 24,610
Amount of regulatory costs not yet approved	53,300		53,300				42,700
Regulatory liabilities	1,734,500		1,734,500				1,594,100
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Regulatory assets	266,000		266,000
Deferred Costs from Generation Asset Sale | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets	16,000		16,000
Other Income, Net	8,700
Deferred Costs from Generation Asset Sale | The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Regulatory assets	149,000		149,000
Deferred Costs from Generation Asset Sale | NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory assets	101,000		101,000
Rate Reduction Bonds | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued						$ 635,700
Revenue Subject to Refund
Regulatory Assets [Line Items]
Regulatory liabilities	36,200		36,200				0
Revenue Subject to Refund | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Regulatory liabilities	9,400		9,400				0
Revenue Subject to Refund | The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Regulatory liabilities	8,300		8,300				0
Revenue Subject to Refund | NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory liabilities	$ 3,700		$ 3,700				$ 0