REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 6,475,200	$ 6,569,900
Less:  Current Portion	1,016,304	1,076,556
Total Long-Term Regulatory Assets	5,458,928	5,493,330
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,566,100	2,794,200
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	756,400	747,100
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	489,700	522,100
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	944,200	765,600
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	883,000	850,500
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	265,300	296,300
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	302,000	314,700
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	121,600	118,400
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	146,900	161,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	2,307,100	2,211,800
Less:  Current Portion	325,268	345,622
Total Long-Term Regulatory Assets	1,981,805	1,866,152
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	540,200	632,300
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	458,100	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	674,200	515,100
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	304,600	246,600
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	265,300	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	33,700	32,100
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	31,000	33,700
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,636,100	1,703,900
Less:  Current Portion	388,408	399,882
Total Long-Term Regulatory Assets	1,247,715	1,304,019
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	638,400	690,000
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	113,800	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	198,100	186,400
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	320,000	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	259,300	270,200
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	57,800	58,600
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	48,700	56,100
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	927,700	989,100
Less:  Current Portion	97,948	115,852
Total Long-Term Regulatory Assets	829,848	873,203
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	249,700	267,600
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	18,700	15,200
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	489,700	522,100
PSNH | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	71,900	64,100
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	76,800	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	4,000	3,900
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 16,900	$ 20,900