REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,547,200	$ 4,468,700
Less:  Current Portion	652,358	602,432
Total Long-Term Regulatory Liabilities	3,894,768	3,866,251
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,665,700	2,685,200
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	667,200	649,600
Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	571,800	448,400
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	128,300	133,500
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	85,100	81,000
CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,900	81,300
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	416,200	389,700
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,459,900	1,459,800
Less:  Current Portion	251,555	266,489
Total Long-Term Regulatory Liabilities	1,208,297	1,193,259
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	992,700	996,100
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	108,600	100,100
CL&P | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	226,700	182,000
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	44,000	43,200
CL&P | CL&P Settlement Agreement and Storm Performance Penalty
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,900	81,300
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	75,000	57,100
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,850,300	1,787,300
Less:  Current Portion	279,299	228,248
Total Long-Term Regulatory Liabilities	1,570,969	1,559,072
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	973,300	984,500
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	388,200	381,000
NSTAR Electric | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	237,200	185,100
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	103,100	107,400
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	41,100	37,800
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	107,400	91,500
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	495,200	501,600
Less:  Current Portion	106,393	120,176
Total Long-Term Regulatory Liabilities	388,838	381,366
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	356,100	359,200
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	19,100	17,200
PSNH | Regulatory Tracking Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	94,900	107,000
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
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	$ 25,100	$ 18,200