REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,715,800	$ 6,569,900
Less:  Current Portion	1,129,093	1,076,556
Total Long-Term Regulatory Assets	4,586,709	5,493,330
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,481,000	2,794,200
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	790,700	747,100
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	478,900	522,100
Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	1,102,700	765,600
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	1,050,500	850,500
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	249,200	296,300
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	297,800	314,700
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	115,000	118,400
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	150,000	161,000
The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Total Regulatory Assets	2,084,800	2,211,800
Less:  Current Portion	371,609	345,622
Total Long-Term Regulatory Assets	1,713,161	1,866,152
The Connecticut Light and Power Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	272,400	632,300
The Connecticut Light and Power Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	470,500	458,900
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 Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	695,600	515,100
The Connecticut Light and Power Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	333,600	246,600
The Connecticut Light and Power Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	249,200	293,100
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	33,600	32,100
The Connecticut Light and Power Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	29,900	33,700
NSTAR Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,579,200	1,703,900
Less:  Current Portion	443,956	399,882
Total Long-Term Regulatory Assets	1,135,231	1,304,019
NSTAR Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	395,500	690,000
NSTAR Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	112,600	110,400
NSTAR Electric Company | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric Company | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	341,300	186,400
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	376,600	332,200
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	255,700	270,200
NSTAR Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	59,800	58,600
NSTAR Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	37,700	56,100
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Total Regulatory Assets	786,400	989,100
Less:  Current Portion	107,169	115,852
Total Long-Term Regulatory Assets	679,182	873,203
Public Service Company of New Hampshire | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	118,900	267,600
Public Service Company of New Hampshire | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	17,500	15,200
Public Service Company of New Hampshire | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	478,900	522,100
Public Service Company of New Hampshire | Storm Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	65,800	64,100
Public Service Company of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	85,400	95,300
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	4,100	3,900
Public Service Company of New Hampshire | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 15,800	$ 20,900