REGULATORY ACCOUNTING - Narrative (Details) - USD ($)		3 Months Ended	12 Months Ended
	May 01, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 27, 2019	Apr. 17, 2019	Apr. 16, 2019	Mar. 26, 2019	Nov. 16, 2018	May 08, 2018
Regulatory Assets [Line Items]
Deferred storm restoration costs			$ 5,514,700,000	$ 5,145,900,000
Amount of regulatory costs not yet approved			146,000,000.0	122,900,000
Other Income, Net			132,777,000	128,366,000	$ 107,913,000
Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs			126,000,000
Goodwill-related
Regulatory Assets [Line Items]
Deferred storm restoration costs			$ 331,500,000	348,400,000
Recovery period			40 years
Remaining amortization			20 years
PSNH
Regulatory Assets [Line Items]
Deferred storm restoration costs			$ 945,800,000	929,500,000
Amount of regulatory costs not yet approved			18,000,000.0	12,200,000
Equity return not recorded on the balance sheet			6,500,000	12,000,000.0
Other Income, Net			19,222,000	27,672,000	9,805,000
PSNH | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs			16,000,000
Other Income, Net		$ 5,200,000	6,300,000	8,700,000
PSNH | Storm Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs						$ 68,500,000			$ 38,100,000
PSNH | Goodwill-related
Regulatory Assets [Line Items]
Deferred storm restoration costs			0	0
CL&P
Regulatory Assets [Line Items]
Deferred storm restoration costs			1,735,900,000	1,630,700,000
Amount of regulatory costs not yet approved			51,800,000	42,100,000
Equity return not recorded on the balance sheet			500,000	700,000
Other Income, Net			17,531,000	22,663,000	22,991,000
CL&P | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs			62,000,000
CL&P | Storm Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs	$ 141,000,000.0						$ 141,000,000.0
Amount of regulatory costs not yet approved								$ 145,500,000		$ 153,000,000
Recovery period	6 years
CL&P | Goodwill-related
Regulatory Assets [Line Items]
Deferred storm restoration costs			0	0
NSTAR Electric
Regulatory Assets [Line Items]
Deferred storm restoration costs			1,535,600,000	1,438,200,000
Amount of regulatory costs not yet approved			55,700,000	49,300,000
Other Income, Net			44,577,000	53,066,000	$ 34,100,000
NSTAR Electric | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs			48,000,000
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Deferred storm restoration costs			$ 284,600,000	$ 299,100,000
Rate Reduction Bonds | PSNH
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued											$ 635,700,000