REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 3,937,100	$ 3,765,400
Less: Current Portion	247,369	128,071
Total Long-Term Regulatory Liabilities	3,689,679	3,637,273
Excess ADIT due to Tax Cuts and Jobs Act
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,885,300	2,882,000
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	517,900	502,100
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	131,000	132,300
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	235,700	136,700
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	68,100	67,100
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	36,500	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	62,600	45,200
The Connecticut Light and Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,196,900	1,151,100
Less: Current Portion	72,183	38,967
Total Long-Term Regulatory Liabilities	1,124,721	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	34,800	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	41,000	34,600
The Connecticut Light and Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	48,100	48,800
The Connecticut Light and Power Company | Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	16,600	0
The Connecticut Light and Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	24,800	12,900
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,631,000	1,594,100
Less: Current Portion	90,222	79,562
Total Long-Term Regulatory Liabilities	1,540,755	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	299,500	293,800
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	110,900	112,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	90,600	77,800
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	20,000	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	15,200	3,700
Public Service Company of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	477,900	450,700
Less: Current Portion	30,023	6,251
Total Long-Term Regulatory Liabilities	447,896	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	34,900	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	29,800	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	6,200	0
Public Service Company of New Hampshire | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 1,900	$ 2,700