REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,468,700	$ 4,240,200
Less:  Current Portion	602,432	389,430
Total Long-Term Regulatory Liabilities	3,866,251	3,850,781
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,685,200	2,778,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	649,600	624,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	133,500	83,600
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	448,400	366,500
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	81,000	76,800
CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	81,300	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	389,700	309,900
The Connecticut Light and Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,459,800	1,342,100
Less:  Current Portion	266,489	137,166
Total Long-Term Regulatory Liabilities	1,193,259	1,204,942
The Connecticut Light and Power Company | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	996,100	1,010,700
The Connecticut Light and Power Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	100,100	98,400
The Connecticut Light and Power Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
The Connecticut Light and Power Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	182,000	148,900
The Connecticut Light and Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	43,200	44,600
The Connecticut Light and Power Company | CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	81,300	0
The Connecticut Light and Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	57,100	39,500
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,787,300	1,715,200
Less:  Current Portion	228,248	164,761
Total Long-Term Regulatory Liabilities	1,559,072	1,550,390
NSTAR Electric Company | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	984,500	1,044,000
NSTAR Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	381,000	363,600
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	107,400	72,500
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	185,100	139,700
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	37,800	32,200
NSTAR Electric Company | CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
NSTAR Electric Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	91,500	63,200
Public Service Company of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	501,600	442,000
Less:  Current Portion	120,176	58,756
Total Long-Term Regulatory Liabilities	381,366	383,183
Public Service Company of New Hampshire | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	359,200	371,500
Public Service Company of New Hampshire | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	17,200	12,900
Public Service Company of New Hampshire | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	107,000	47,800
Public Service Company of New Hampshire | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company of New Hampshire | CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company of New Hampshire | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 18,200	$ 9,800