SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 50,149	$ 29,022	$ 83,938	$ 50,641
Equity securities, unrealized gain (loss)	17,600	9,700	17,600	9,700
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	14,600	7,500	29,800	15,500
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	10,900	8,000	20,600	14,600
Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	22,900	13,300	27,500	17,900
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(300)	(1,700)	400	(1,000)
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	1,900	1,900	5,400	3,600
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	100	0	200	0
The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net	7,063	4,221	13,623	7,489
The Connecticut Light and Power Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	2,400	400	5,300	900
The Connecticut Light and Power Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	3,300	2,900	6,100	4,800
The Connecticut Light and Power Company | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
The Connecticut Light and Power Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	400	(400)	200	(600)
The Connecticut Light and Power Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	1,000	1,300	2,000	2,400
The Connecticut Light and Power Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net	14,269	7,096	26,870	15,445
NSTAR Electric Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	9,500	4,800	17,900	9,800
NSTAR Electric Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	3,900	2,200	7,300	4,100
NSTAR Electric Company | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	400	(100)	400	(100)
NSTAR Electric Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	300	(100)	900	1,100
NSTAR Electric Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	200	300	400	500
NSTAR Electric Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net	3,409	1,891	8,159	4,703
Public Service Company of New Hampshire | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	2,100	1,500	4,400	3,100
Public Service Company of New Hampshire | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
Public Service Company of New Hampshire | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
Public Service Company of New Hampshire | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	0	(200)	100	500
Public Service Company of New Hampshire | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	1,300	600	3,700	1,100
Public Service Company of New Hampshire | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 0	$ 0	$ 0	$ 0