REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,534,600	$ 5,514,700
Less: Current Portion	805,050	651,112
Total Long-Term Regulatory Assets	4,729,531	4,863,639
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,306,700	2,382,900
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	731,800	725,800
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	543,700	565,300
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	486,600	540,600
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	590,700	411,500
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	321,900	334,500
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	323,100	331,500
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	102,500	97,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	127,600	125,400
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	1,837,000	1,735,900
Less: Current Portion	349,177	178,607
Total Long-Term Regulatory Assets	1,487,764	1,557,261
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	493,500	539,000
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	462,400	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	251,900	274,600
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	251,000	78,300
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	322,000	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	31,700	30,800
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	24,500	25,200
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,514,400	1,535,600
Less: Current Portion	296,366	285,591
Total Long-Term Regulatory Assets	1,217,978	1,250,029
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	615,600	629,800
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	109,100	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	176,900	200,600
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	222,400	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	277,400	284,600
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	53,800	50,300
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	59,200	55,200
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	949,200	945,800
Less: Current Portion	110,267	84,053
Total Long-Term Regulatory Assets	838,931	861,672
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	209,900	218,200
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	12,600	12,800
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	543,700	565,300
PSNH | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	57,800	65,400
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	106,700	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	3,700	3,600
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 14,800	$ 14,700