REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,121,900	$ 4,019,200
Less: Current Portion	442,004	361,152
Total Long-Term Regulatory Liabilities	3,679,927	3,658,042
EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,817,700	2,844,600
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	590,500	559,800
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	83,900	84,500
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	417,700	325,100
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	75,500	73,200
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	136,600	132,000
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,338,800	1,247,800
Less: Current Portion	155,239	82,763
Total Long-Term Regulatory Liabilities	1,183,643	1,164,991
CL&P | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,016,700	1,022,800
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	84,600	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	156,400	94,800
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	45,300	46,000
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	35,800	19,600
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,712,500	1,725,800
Less: Current Portion	185,262	209,180
Total Long-Term Regulatory Liabilities	1,527,194	1,516,585
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,056,500	1,071,200
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	345,700	330,600
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	72,400	72,200
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	154,900	165,600
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	30,200	27,200
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	52,800	59,000
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	489,400	479,200
Less: Current Portion	83,588	65,766
Total Long-Term Regulatory Liabilities	405,773	413,381
PSNH | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	390,100	392,800
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,100	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	78,600	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	$ 8,600	$ 13,100