REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,240,200	$ 4,019,200
Less:  Current Portion	389,430	361,152
Total Long-Term Regulatory Liabilities	3,850,781	3,658,042
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,778,600	2,844,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	624,800	559,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	83,600	84,500
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	366,500	325,100
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	76,800	73,200
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	309,900	132,000
The Connecticut Light and Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,342,100	1,247,800
Less:  Current Portion	137,166	82,763
Total Long-Term Regulatory Liabilities	1,204,942	1,164,991
The Connecticut Light and Power Company | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,010,700	1,022,800
The Connecticut Light and Power Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	98,400	64,600
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	148,900	94,800
The Connecticut Light and Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	44,600	46,000
The Connecticut Light and Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	39,500	19,600
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,715,200	1,725,800
Less:  Current Portion	164,761	209,180
Total Long-Term Regulatory Liabilities	1,550,390	1,516,585
NSTAR Electric Company | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,044,000	1,071,200
NSTAR Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	363,600	330,600
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	72,500	72,200
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	139,700	165,600
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	32,200	27,200
NSTAR Electric Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	63,200	59,000
Public Service Company of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	442,000	479,200
Less:  Current Portion	58,756	65,766
Total Long-Term Regulatory Liabilities	383,183	413,381
Public Service Company of New Hampshire | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	371,500	392,800
Public Service Company of New Hampshire | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,900	16,300
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	47,800	57,000
Public Service Company of New Hampshire | AFUDC - Transmission
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	$ 9,800	$ 13,100