REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended
	May 01, 2019	Mar. 31, 2019	Mar. 31, 2018	Apr. 17, 2019	Mar. 26, 2019	Dec. 31, 2018	Nov. 16, 2018
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		$ 129,500				$ 122,900
Regulatory assets		5,096,700				5,145,900
Other income, net		30,985	$ 33,789
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		13,900				12,200
Regulatory assets		932,000				929,500
Other income, net		7,022	4,749
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		41,900				42,100
Regulatory assets		1,644,400				1,630,700
Other income, net		3,880	6,560
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		52,800				49,300
Regulatory assets		1,408,600				$ 1,438,200
Other income, net		11,086	$ 12,601
Storm Costs | PSNH
Regulatory Assets [Line Items]
Regulatory assets					$ 38,100
Storm Costs | CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		145,500					$ 153,000
Storm Costs | CL&P | Subsequent Event
Regulatory Assets [Line Items]
Regulatory assets				$ 141,000
Recovery period	6 years
Deferred Costs from Generation Asset Sale | PSNH
Regulatory Assets [Line Items]
Other income, net		$ 5,200