REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 6,433,300	$ 6,569,900
Less:  Current Portion	1,186,851	1,076,556
Total Long-Term Regulatory Assets	5,246,381	5,493,330
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,625,100	2,794,200
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	755,400	747,100
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	500,500	522,100
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	745,900	765,600
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	950,900	850,500
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	279,700	296,300
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	306,200	314,700
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	118,200	118,400
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	151,400	161,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	2,204,400	2,211,800
Less:  Current Portion	458,609	345,622
Total Long-Term Regulatory Assets	1,745,793	1,866,152
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	551,700	632,300
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	460,500	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	500,200	515,100
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	347,500	246,600
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	279,700	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	33,200	32,100
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	31,600	33,700
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,684,400	1,703,900
Less:  Current Portion	448,030	399,882
Total Long-Term Regulatory Assets	1,236,379	1,304,019
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	653,900	690,000
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	112,200	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	169,500	186,400
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	378,800	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	262,900	270,200
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	55,800	58,600
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	51,300	56,100
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	950,400	989,100
Less:  Current Portion	99,474	115,852
Total Long-Term Regulatory Assets	850,912	873,203
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	255,100	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	500,500	522,100
PSNH | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	76,200	64,100
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	79,100	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	$ 18,100	$ 20,900