REGULATORY ACCOUNTING - Narrative (Details) - USD ($)						1 Months Ended	12 Months Ended
	Dec. 23, 2020	Dec. 17, 2020	Dec. 15, 2020	Oct. 30, 2020	Oct. 07, 2020	Sep. 30, 2019	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	May 08, 2018
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							$ 196,900,000		$ 146,000,000.0
Regulatory assets							6,569,900,000		5,514,700,000
Deferred costs related to uncollectible expense incurred at our natural gas distribution segment							15,800,000
Deferred costs related to facilities and fleet cleaning, sanitizing costs and supplies for personal protective equipment							8,200,000
NHPUC
Regulatory Assets [Line Items]
After-tax benefit to earnings from settlement agreement							11,000,000.0
Increase in divestiture-related costs incurred to be recovered		$ 12,000,000.0				$ 12,000,000.0
Divestiture costs incurred to be recovered								$ 635,700,000
Recovery period		1 year
DPU
Regulatory Assets [Line Items]
Authorized regulatory ROE					9.70%
Authorized common equity ratio					53.25%
FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers							39,100,000
Regulatory Assets | NHPUC
Regulatory Assets [Line Items]
Divestiture costs incurred to be recovered							12,000,000.0
Long-Term Derivative Assets | NHPUC
Regulatory Assets [Line Items]
Divestiture costs incurred to be recovered							(12,000,000.0)
COVID 19
Regulatory Assets [Line Items]
Deferred costs							24,000,000.0
Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							591,000,000
Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets							$ 314,700,000		331,500,000
Recovery period							40 years
Remaining amortization							19 years
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory assets							$ 765,600,000		540,600,000
Storm Restoration Costs, Net | Tropical Storm Isaias
Regulatory Assets [Line Items]
Regulatory assets							245,000,000
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							4,300,000		18,000,000.0
Regulatory assets							989,100,000		945,800,000
Deferred costs related to uncollectible expense incurred at our natural gas distribution segment							600,000
Deferred costs related to facilities and fleet cleaning, sanitizing costs and supplies for personal protective equipment							500,000
Equity return not recorded on the balance sheet							5,100,000		6,500,000
Public Service Company of New Hampshire | NHPUC
Regulatory Assets [Line Items]
Rate increase approved			$ 45,000,000.0
Revenue requirement	$ 10,600,000
Authorized regulatory ROE			9.30%
Authorized common equity ratio			54.40%
After-tax benefit to earnings from settlement agreement							7,200,000
Public Service Company of New Hampshire | FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers							3,100,000
Public Service Company of New Hampshire | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							35,000,000
Public Service Company of New Hampshire | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets							0		0
Public Service Company of New Hampshire | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory assets							64,100,000		65,400,000
The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							84,100,000		51,800,000
Regulatory assets							2,211,800,000		1,735,900,000
Deferred costs related to uncollectible expense incurred at our natural gas distribution segment							3,000,000.0
Deferred costs related to facilities and fleet cleaning, sanitizing costs and supplies for personal protective equipment							1,700,000
Equity return not recorded on the balance sheet							200,000		500,000
The Connecticut Light and Power Company | FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers							21,400,000
The Connecticut Light and Power Company | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							390,000,000
The Connecticut Light and Power Company | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets							0		0
The Connecticut Light and Power Company | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory assets							515,100,000		274,600,000
The Connecticut Light and Power Company | Storm Restoration Costs, Net | Tropical Storm Isaias
Regulatory Assets [Line Items]
Regulatory assets							228,000,000
NSTAR Electric Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							69,800,000		55,700,000
Regulatory assets							1,703,900,000		1,535,600,000
Deferred costs related to uncollectible expense incurred at our natural gas distribution segment							6,800,000
Deferred costs related to facilities and fleet cleaning, sanitizing costs and supplies for personal protective equipment							5,100,000
NSTAR Electric Company | FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers							14,600,000
NSTAR Electric Company | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved							166,000,000
NSTAR Electric Company | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets							270,200,000		284,600,000
NSTAR Electric Company | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory assets							$ 186,400,000		$ 200,600,000
NSTAR Gas | DPU
Regulatory Assets [Line Items]
Rate increase approved				$ 23,000,000.0
Authorized regulatory ROE				9.90%
Authorized common equity ratio				54.77%
After-tax benefit to earnings from settlement agreement				$ 2,700,000
Rate increase requested				$ 38,000,000.0
Performance-based ratemaking plan period				10 years
Rate Reduction Bonds | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued										$ 635,700,000