REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,514,700	$ 5,145,900
Less: Current Portion	651,112	514,779
Total Long-Term Regulatory Assets	4,863,639	4,631,137
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,382,900	1,914,800
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	725,800	728,600
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	565,300	608,400
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	540,600	576,000
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	411,500	316,000
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	334,500	356,500
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	331,500	348,400
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	97,200	89,200
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	125,400	208,000
CL&P
Regulatory Assets [Line Items]
Total Regulatory Assets	1,735,900	1,630,700
Less: Current Portion	178,607	125,155
Total Long-Term Regulatory Assets	1,557,261	1,505,488
CL&P | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	539,000	424,700
CL&P | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	458,800	454,400
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	274,600	302,600
CL&P | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	78,300	33,200
CL&P | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	329,200	356,500
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	30,800	32,300
CL&P | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	25,200	27,000
NSTAR Electric
Regulatory Assets [Line Items]
Total Regulatory Assets	1,535,600	1,438,200
Less: Current Portion	285,591	241,747
Total Long-Term Regulatory Assets	1,250,029	1,196,512
NSTAR Electric | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	629,800	544,400
NSTAR Electric | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	108,000	105,900
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	200,600	212,900
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	207,100	169,100
NSTAR Electric | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	284,600	299,100
NSTAR Electric | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	50,300	42,200
NSTAR Electric | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	55,200	64,600
PSNH
Regulatory Assets [Line Items]
Total Regulatory Assets	945,800	929,500
Less: Current Portion	84,053	67,228
Total Long-Term Regulatory Assets	861,672	862,288
PSNH | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	218,200	169,600
PSNH | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	12,800	8,300
PSNH | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	565,300	608,400
PSNH | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	65,400	60,500
PSNH | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	65,800	67,300
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	3,600	3,300
PSNH | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 14,700	$ 12,100