SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 128,366	$ 107,913	$ 64,505
Interest Income	18,100	8,300	11,000
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	44,000	34,400	26,200
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	60,800	29,900	18,600
Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	3,800	27,400	200
Equity in Earnings of Subsidiaries	17,600	9,700	1,700
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(4,000)	7,500	8,500
Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	5,100	0	0
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	600	400	0
The Connecticut Light And Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net	22,663	22,991	14,238
The Connecticut Light And Power Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	9,500	1,800	700
The Connecticut Light And Power Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	12,200	12,100	6,300
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	12,200	12,100	6,300
The Connecticut Light And Power Company | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	100	0	100
The Connecticut Light And Power Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(3,000)	4,500	(1,500)
The Connecticut Light And Power Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	3,700	4,600	8,600
The Connecticut Light And Power Company | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0
The Connecticut Light And Power Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	200	0	0
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net	53,066	34,100	21,263
NSTAR Electric Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	36,000	19,200	10,500
NSTAR Electric Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	15,600	10,200	10,200
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	15,600	10,200	10,200
NSTAR Electric Company | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	700	300	300
NSTAR Electric Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(500)	2,600	(300)
NSTAR Electric Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	800	1,800	600
NSTAR Electric Company | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	500	0	0
NSTAR Electric Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0
Public Service Company Of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net	27,672	9,805	7,563
Public Service Company Of New Hampshire | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	9,900	5,900	6,200
Public Service Company Of New Hampshire | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	300
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	0	0	300
Public Service Company Of New Hampshire | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0
Public Service Company Of New Hampshire | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(800)	1,600	(700)
Public Service Company Of New Hampshire | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	14,100	2,200	1,800
Public Service Company Of New Hampshire | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	4,400	0	0
Public Service Company Of New Hampshire | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 100	$ 100	$ 0