SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 33,789	$ 21,619
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	15,200	8,000
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	9,700	6,700
Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	4,600	4,600
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	700	700
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	3,500	1,600
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	100	0
Total Other Income, Net
Summary of Accounting Policies [Line Items]
Other Income, Net	33,800	21,600
The Connecticut Light And Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net	6,560	3,267
The Connecticut Light And Power Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	3,000	500
The Connecticut Light And Power Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	2,800	2,000
The Connecticut Light And Power Company | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
The Connecticut Light And Power Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(100)	(300)
The Connecticut Light And Power Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	900	1,100
The Connecticut Light And Power Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
The Connecticut Light And Power Company | Total Other Income, Net
Summary of Accounting Policies [Line Items]
Other Income, Net	6,600	3,300
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net	12,601	8,349
NSTAR Electric Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	8,400	5,000
NSTAR Electric Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	3,400	1,900
NSTAR Electric Company | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	100
NSTAR Electric Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	600	1,200
NSTAR Electric Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	200	100
NSTAR Electric Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
NSTAR Electric Company | Total Other Income, Net
Summary of Accounting Policies [Line Items]
Other Income, Net	12,600	8,300
Public Service Company Of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net	4,749	2,811
Public Service Company Of New Hampshire | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	2,300	1,600
Public Service Company Of New Hampshire | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
Public Service Company Of New Hampshire | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
Public Service Company Of New Hampshire | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	0	700
Public Service Company Of New Hampshire | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	2,400	500
Public Service Company Of New Hampshire | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
Public Service Company Of New Hampshire | Total Other Income, Net
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 4,700	$ 2,800