REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,383,800	$ 4,240,200
Less:  Current Portion	520,833	389,430
Total Long-Term Regulatory Liabilities	3,863,031	3,850,781
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,762,300	2,778,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	644,200	624,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	79,100	83,600
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	467,000	366,500
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	77,800	76,800
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	353,400	309,900
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,406,000	1,342,100
Less:  Current Portion	202,362	137,166
Total Long-Term Regulatory Liabilities	1,203,616	1,204,942
CL&P | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,008,100	1,010,700
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	105,100	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	175,200	148,900
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	44,300	44,600
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	73,300	39,500
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,746,900	1,715,200
Less:  Current Portion	190,546	164,761
Total Long-Term Regulatory Liabilities	1,556,422	1,550,390
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,037,300	1,044,000
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	372,100	363,600
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	68,700	72,500
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	165,200	139,700
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	33,500	32,200
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	70,100	63,200
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	434,500	442,000
Less:  Current Portion	48,023	58,756
Total Long-Term Regulatory Liabilities	386,455	383,183
PSNH | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	366,500	371,500
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	16,400	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	40,400	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	$ 11,200	$ 9,800