REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,051,500	$ 3,765,400
Less: Current Portion	344,708	128,071
Total Long-Term Regulatory Liabilities	3,706,792	3,637,273
Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,876,900	2,882,000
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	516,400	502,100
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	127,100	132,300
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	345,600	136,700
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	68,600	67,100
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	36,200	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	80,700	45,200
The Connecticut Light and Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,257,800	1,151,100
Less: Current Portion	126,574	38,967
Total Long-Term Regulatory Liabilities	1,131,234	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	1,026,300	1,031,600
The Connecticut Light and Power Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	37,800	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	111,900	34,600
The Connecticut Light and Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	47,800	48,800
The Connecticut Light and Power Company | Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	8,300	0
The Connecticut Light and Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	25,700	12,900
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,734,500	1,594,100
Less: Current Portion	168,225	79,562
Total Long-Term Regulatory Liabilities	1,566,285	1,514,451
NSTAR Electric Company | Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,100,500	1,087,900
NSTAR Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	305,700	293,800
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	107,200	112,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	168,300	77,800
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	20,800	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	28,300	3,700
Public Service Company of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	473,500	450,700
Less: Current Portion	39,661	6,251
Total Long-Term Regulatory Liabilities	433,822	444,397
Public Service Company of New Hampshire | Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	397,400	405,100
Public Service Company of New Hampshire | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	24,900	37,900
Public Service Company of New Hampshire | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	500	0
Public Service Company of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	38,500	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	9,400	0
Public Service Company of New Hampshire | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 2,800	$ 2,700