REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,019,700	$ 3,979,700
Less: Current Portion	379,850	370,230
Total Long-Term Regulatory Liabilities	3,639,792	3,609,475
EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,860,700	2,883,000
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	544,900	521,000
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	83,800	91,200
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	333,300	309,000
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	71,700	70,700
Revenue Subject to Refund due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	29,200	24,600
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	96,100	80,200
CL&P
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,265,800	1,231,800
Less: Current Portion	112,414	109,614
Total Long-Term Regulatory Liabilities	1,153,360	1,122,157
CL&P | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,028,700	1,031,000
CL&P | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	51,000	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	110,000	89,500
CL&P | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	46,700	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	29,400	24,000
NSTAR Electric
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,685,800	1,703,900
Less: Current Portion	170,970	190,620
Total Long-Term Regulatory Liabilities	1,514,845	1,513,279
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,090,000	1,103,700
NSTAR Electric | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	319,400	307,100
NSTAR Electric | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	70,900	76,900
NSTAR Electric | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	141,900	163,700
NSTAR Electric | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	25,000	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	38,600	29,200
PSNH
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	471,200	483,600
Less: Current Portion	54,183	55,526
Total Long-Term Regulatory Liabilities	416,999	428,069
PSNH | EDIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	395,400	396,400
PSNH | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	21,400	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	31,200	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	19,100	12,600
PSNH | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,100	$ 4,200