REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 6,569,900	$ 5,514,700
Less:  Current Portion	1,076,556	651,112
Total Long-Term Regulatory Assets	5,493,330	4,863,639
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,794,200	2,382,900
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	747,100	725,800
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	522,100	565,300
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	765,600	540,600
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	850,500	411,500
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	296,300	334,500
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	314,700	331,500
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	118,400	97,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	161,000	125,400
The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Total Regulatory Assets	2,211,800	1,735,900
Less:  Current Portion	345,622	178,607
Total Long-Term Regulatory Assets	1,866,152	1,557,261
The Connecticut Light and Power Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	632,300	539,000
The Connecticut Light and Power Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	458,900	458,800
The Connecticut Light and Power Company | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
The Connecticut Light and Power Company | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	515,100	274,600
The Connecticut Light and Power Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	246,600	78,300
The Connecticut Light and Power Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	293,100	329,200
The Connecticut Light and Power Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
The Connecticut Light and Power Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	32,100	30,800
The Connecticut Light and Power Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	33,700	25,200
NSTAR Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,703,900	1,535,600
Less:  Current Portion	399,882	285,591
Total Long-Term Regulatory Assets	1,304,019	1,250,029
NSTAR Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	690,000	629,800
NSTAR Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	110,400	108,000
NSTAR Electric Company | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric Company | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	186,400	200,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	332,200	207,100
NSTAR Electric Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	270,200	284,600
NSTAR Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	58,600	50,300
NSTAR Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	56,100	55,200
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Total Regulatory Assets	989,100	945,800
Less:  Current Portion	115,852	84,053
Total Long-Term Regulatory Assets	873,203	861,672
Public Service Company of New Hampshire | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	267,600	218,200
Public Service Company of New Hampshire | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	15,200	12,800
Public Service Company of New Hampshire | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	522,100	565,300
Public Service Company of New Hampshire | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	64,100	65,400
Public Service Company of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	95,300	65,800
Public Service Company of New Hampshire | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company of New Hampshire | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company of New Hampshire | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	3,900	3,600
Public Service Company of New Hampshire | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 20,900	$ 14,700