REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 3,909,700	$ 3,765,400
Less: Current Portion	205,970	128,071
Total Long-Term Regulatory Liabilities	3,703,735	3,637,273
Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,895,900	2,882,000
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	522,800	502,100
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	128,000	132,300
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	216,800	136,700
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	67,600	67,100
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	26,000	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	52,600	45,200
The Connecticut Light And Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,203,300	1,151,100
Less: Current Portion	66,949	38,967
Total Long-Term Regulatory Liabilities	1,136,361	1,112,136
The Connecticut Light And Power Company | Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,031,600	1,031,600
The Connecticut Light And Power Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	34,400	23,200
The Connecticut Light And Power Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
The Connecticut Light And Power Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	55,900	34,600
The Connecticut Light And Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	48,500	48,800
The Connecticut Light And Power Company | Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,500	0
The Connecticut Light And Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	20,400	12,900
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,619,300	1,594,100
Less: Current Portion	89,230	79,562
Total Long-Term Regulatory Liabilities	1,530,103	1,514,451
NSTAR Electric Company | Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,091,100	1,087,900
NSTAR Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	298,500	293,800
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	109,000	112,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	89,200	77,800
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	19,100	18,300
NSTAR Electric Company | Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	3,700	0
NSTAR Electric Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	8,700	3,700
Public Service Company Of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	465,200	450,700
Less: Current Portion	15,607	6,251
Total Long-Term Regulatory Liabilities	449,588	444,397
Public Service Company Of New Hampshire | Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	405,100	405,100
Public Service Company Of New Hampshire | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	39,000	37,900
Public Service Company Of New Hampshire | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company Of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	15,100	5,000
Public Service Company Of New Hampshire | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company Of New Hampshire | Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	3,100	0
Public Service Company Of New Hampshire | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 2,900	$ 2,700