REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,108,100	$ 4,019,200
Less: Current Portion	439,859	361,152
Total Long-Term Regulatory Liabilities	3,668,158	3,658,042
EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,831,200	2,844,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	579,300	559,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	80,100	84,500
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	412,500	325,100
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	74,400	73,200
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	130,600	132,000
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,299,000	1,247,800
Less: Current Portion	119,283	82,763
Total Long-Term Regulatory Liabilities	1,179,663	1,164,991
CL&P | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,019,800	1,022,800
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	77,400	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	121,700	94,800
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	45,700	46,000
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	34,400	19,600
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,735,600	1,725,800
Less: Current Portion	219,872	209,180
Total Long-Term Regulatory Liabilities	1,515,732	1,516,585
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,063,900	1,071,200
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	337,200	330,600
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	68,600	72,200
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	189,800	165,600
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	28,700	27,200
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	47,400	59,000
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	481,000	479,200
Less: Current Portion	69,042	65,766
Total Long-Term Regulatory Liabilities	411,959	413,381
PSNH | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	391,400	392,800
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	14,800	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	64,100	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	$ 10,700	$ 13,100