REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,012,800	$ 3,979,700
Less: Current Portion	385,442	370,230
Total Long-Term Regulatory Liabilities	3,627,389	3,609,475
EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,871,400	2,883,000
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	535,000	521,000
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	87,200	91,200
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	332,500	309,000
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	71,400	70,700
Revenue Subject to Refund due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	27,000	24,600
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	88,300	80,200
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,232,800	1,231,800
Less: Current Portion	106,489	109,614
Total Long-Term Regulatory Liabilities	1,126,279	1,122,157
CL&P | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,030,700	1,031,000
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	46,200	39,900
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	79,500	89,500
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	47,100	47,400
CL&P | Revenue Subject to Refund due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
CL&P | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	29,300	24,000
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,708,600	1,703,900
Less: Current Portion	193,959	190,620
Total Long-Term Regulatory Liabilities	1,514,557	1,513,279
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,096,700	1,103,700
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	313,400	307,100
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	73,600	76,900
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	166,700	163,700
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	24,300	23,300
NSTAR Electric | Revenue Subject to Refund due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
NSTAR Electric | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	33,900	29,200
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	475,600	483,600
Less: Current Portion	45,655	55,526
Total Long-Term Regulatory Liabilities	429,932	428,069
PSNH | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	394,600	396,400
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	22,100	22,100
PSNH | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	39,900	48,300
PSNH | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
PSNH | Revenue Subject to Refund due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	15,800	12,600
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 3,200	$ 4,200