REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,176,200	$ 4,019,200
Less: Current Portion	464,673	361,152
Total Long-Term Regulatory Liabilities	3,711,488	3,658,042
EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,805,300	2,844,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	610,400	559,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	79,500	84,500
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	445,700	325,100
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	77,100	73,200
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	158,200	132,000
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,361,200	1,247,800
Less: Current Portion	164,946	82,763
Total Long-Term Regulatory Liabilities	1,196,255	1,164,991
CL&P | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,013,900	1,022,800
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	93,000	64,600
CL&P | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
CL&P | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	171,100	94,800
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	45,000	46,000
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	38,200	19,600
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,745,500	1,725,800
Less: Current Portion	210,744	209,180
Total Long-Term Regulatory Liabilities	1,534,760	1,516,585
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,050,100	1,071,200
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	354,800	330,600
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	68,700	72,200
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	181,900	165,600
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	32,100	27,200
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	57,900	59,000
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	467,700	479,200
Less: Current Portion	61,457	65,766
Total Long-Term Regulatory Liabilities	406,211	413,381
PSNH | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	388,700	392,800
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,900	16,300
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	56,200	57,000
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 9,900	$ 13,100