REGULATORY ACCOUNTING - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Oct. 01, 2021	Sep. 01, 2021	May 06, 2021	May 08, 2018
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 252,500,000	$ 196,900,000
Regulatory assets	5,715,800,000	6,569,900,000
Deferred costs related to uncollectible expense	33,000,000	15,800,000
Regulatory Liabilities	602,432,000	389,430,000
FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	39,100,000
COVID 19
Regulatory Assets [Line Items]
Deferred costs	39,800,000	24,000,000
Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	1,010,000,000.00
Deferred Storm Restoration Costs | Tropical Storm Isaias
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 251,000,000
Goodwill-related
Regulatory Assets [Line Items]
Recovery period	40 years
Remaining amortization	18 years
Regulatory assets	$ 297,800,000	314,700,000
Storm Costs, Net
Regulatory Assets [Line Items]
Regulatory assets	1,102,700,000	765,600,000
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	3,400,000	4,300,000
Regulatory assets	786,400,000	989,100,000
Equity return not recorded on the balance sheet	5,000,000	5,100,000
Regulatory Liabilities	120,176,000	58,756,000
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	61,000,000
Public Service Company of New Hampshire | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets	0	0
Public Service Company of New Hampshire | Storm Costs, Net
Regulatory Assets [Line Items]
Regulatory assets	65,800,000	64,100,000
The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	114,900,000	84,100,000
Regulatory assets	2,084,800,000	2,211,800,000
Deferred costs related to uncollectible expense	19,000,000	4,700,000
Equity return not recorded on the balance sheet		200,000
Regulatory Liabilities	266,489,000	137,166,000
The Connecticut Light and Power Company | CL&P Rate Adjustment Mechanisms (RAM)
Regulatory Assets [Line Items]
Recovery of cumulative under-recoveries		193,000,000
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 | Unfavorable Regulatory Action
Regulatory Assets [Line Items]
Regulatory Liabilities	75,000,000
Loss contingency, estimate of possible earnings impact, non-compliance with performance standards				$ 28,400,000	$ 28,400,000
Customer credits			$ 65,000,000
Customer assistance initiatives			$ 10,000,000
The Connecticut Light and Power Company | Deferred Pre-Staging And Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	232,000,000	344,000,000
The Connecticut Light and Power Company | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	643,000,000
The Connecticut Light and Power Company | Deferred Storm Restoration Costs | Tropical Storm Isaias
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	234,000,000
The Connecticut Light and Power Company | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets	0	0
The Connecticut Light and Power Company | Storm Costs, Net
Regulatory Assets [Line Items]
Regulatory assets	695,600,000	515,100,000
NSTAR Electric Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	85,000,000	69,800,000
Regulatory assets	1,579,200,000	1,703,900,000
Deferred costs related to uncollectible expense	11,200,000	11,900,000
Regulatory Liabilities	228,248,000	164,761,000
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	308,000,000
Amount of regulatory recovery not yet approved		7,200,000
Pre-tax benefit to earnings from settlement agreement	15,600,000	15,600,000
NSTAR Electric Company | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets	255,700,000	270,200,000
NSTAR Electric Company | Storm Costs, Net
Regulatory Assets [Line Items]
Regulatory assets	$ 341,300,000	$ 186,400,000
Rate Reduction Bonds | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued						$ 635,700,000