REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,520,500	$ 4,240,200
Less:  Current Portion	612,931	389,430
Total Long-Term Regulatory Liabilities	3,907,582	3,850,781
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,737,100	2,778,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	676,700	624,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	65,900	83,600
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	484,500	366,500
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	79,600	76,800
CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	103,400	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	373,300	309,900
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,524,800	1,342,100
Less:  Current Portion	311,269	137,166
Total Long-Term Regulatory Liabilities	1,213,469	1,204,942
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,001,500	1,010,700
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	117,000	98,400
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
CL&P | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	208,900	148,900
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	43,600	44,600
CL&P | CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	103,400	0
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	50,400	39,500
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,767,400	1,715,200
Less:  Current Portion	205,771	164,761
Total Long-Term Regulatory Liabilities	1,561,572	1,550,390
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,025,600	1,044,000
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	384,100	363,600
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	56,500	72,500
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	180,600	139,700
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	36,000	32,200
NSTAR Electric | CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	84,600	63,200
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	486,200	442,000
Less:  Current Portion	96,817	58,756
Total Long-Term Regulatory Liabilities	389,426	383,183
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	363,900	371,500
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	19,000	12,900
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	87,500	47,800
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 15,800	$ 9,800