REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,646,100	$ 5,514,700
Less: Current Portion	762,883	651,112
Total Long-Term Regulatory Assets	4,883,165	4,863,639
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,256,100	2,382,900
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	731,200	725,800
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	532,900	565,300
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	739,600	540,600
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	531,800	411,500
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	311,800	334,500
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	318,900	331,500
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	105,800	97,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	118,000	125,400
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	2,006,500	1,735,900
Less: Current Portion	273,792	178,607
Total Long-Term Regulatory Assets	1,732,739	1,557,261
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	483,300	539,000
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	459,800	458,800
CL&P | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
CL&P | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	510,700	274,600
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	174,400	78,300
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	311,800	329,200
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	32,300	30,800
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	34,200	25,200
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,472,700	1,535,600
Less: Current Portion	278,604	285,591
Total Long-Term Regulatory Assets	1,194,092	1,250,029
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	601,400	629,800
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	110,000	108,000
NSTAR Electric | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	171,400	200,600
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	203,000	207,100
NSTAR Electric | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	273,800	284,600
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	56,200	50,300
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	56,900	55,200
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	912,400	945,800
Less: Current Portion	103,570	84,053
Total Long-Term Regulatory Assets	808,784	861,672
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	205,700	218,200
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	13,700	12,800
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	532,900	565,300
PSNH | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	57,500	65,400
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	82,300	65,800
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	3,800	3,600
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 16,500	$ 14,700