REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 6,512,600	$ 6,569,900
Less:  Current Portion	1,081,074	1,076,556
Total Long-Term Regulatory Assets	5,431,459	5,493,330
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,733,600	2,794,200
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	751,700	747,100
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	511,300	522,100
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	789,500	765,600
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	856,100	850,500
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	293,400	296,300
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	310,500	314,700
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	114,900	118,400
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	151,600	161,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	2,250,000	2,211,800
Less:  Current Portion	386,140	345,622
Total Long-Term Regulatory Assets	1,863,935	1,866,152
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	618,500	632,300
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	459,100	458,900
CL&P | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	529,800	515,100
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	284,300	246,600
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	293,100	293,100
CL&P | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	32,600	32,100
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	32,600	33,700
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,704,400	1,703,900
Less:  Current Portion	430,361	399,882
Total Long-Term Regulatory Assets	1,273,997	1,304,019
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	674,200	690,000
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	111,400	110,400
NSTAR Electric | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	184,300	186,400
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	360,500	332,200
NSTAR Electric | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	266,500	270,200
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	53,800	58,600
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	53,700	56,100
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	977,200	989,100
Less:  Current Portion	107,322	115,852
Total Long-Term Regulatory Assets	869,904	873,203
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	262,300	267,600
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	17,400	15,200
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	511,300	522,100
PSNH | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	75,400	64,100
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	87,500	95,300
PSNH | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
PSNH | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	3,900	3,900
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 19,400	$ 20,900