REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,412,600	$ 4,240,200
Less:  Current Portion	532,467	389,430
Total Long-Term Regulatory Liabilities	3,880,131	3,850,781
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,748,600	2,778,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	657,100	624,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	70,400	83,600
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	474,700	366,500
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	78,700	76,800
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	383,100	309,900
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,427,100	1,342,100
Less:  Current Portion	219,813	137,166
Total Long-Term Regulatory Liabilities	1,207,289	1,204,942
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,005,400	1,010,700
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	108,700	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	190,300	148,900
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	43,900	44,600
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	78,800	39,500
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,725,100	1,715,200
Less:  Current Portion	172,988	164,761
Total Long-Term Regulatory Liabilities	1,552,053	1,550,390
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,030,500	1,044,000
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	374,400	363,600
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	60,200	72,500
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	147,700	139,700
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	34,800	32,200
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	77,500	63,200
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	446,600	442,000
Less:  Current Portion	61,891	58,756
Total Long-Term Regulatory Liabilities	384,675	383,183
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	364,200	371,500
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	17,100	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	51,000	47,800
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 14,300	$ 9,800