SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 161,282	$ 108,590	$ 132,777
Unrealized gain associated with investment	14,200	14,200	42,200
Renewable Energy Fund
Summary of Accounting Policies [Line Items]
Other-than-temporary impairment		2,800
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	84,400	44,400	31,300
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	37,300	42,000	45,000
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	14,200	14,200	42,200
Unrealized gain associated with investment	2,100	2,400	20,400
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(200)	1,100	800
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	25,600	4,800	12,800
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	2,100	700
The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net	30,187	20,774	17,531
The Connecticut Light and Power Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	15,200	3,800	500
The Connecticut Light and Power Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	7,700	13,800	13,200
The Connecticut Light and Power Company | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	100
The Connecticut Light and Power Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	1,300	1,100	2,300
The Connecticut Light and Power Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	5,900	2,000	1,500
The Connecticut Light and Power Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	100	100	(100)
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net	74,844	52,017	44,577
NSTAR Electric Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	40,200	29,300	23,500
NSTAR Electric Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	20,400	21,500	19,800
NSTAR Electric Company | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	400	400	700
NSTAR Electric Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	100	(800)	(400)
NSTAR Electric Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	13,400	900	700
NSTAR Electric Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	300	700	300
Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net	14,565	13,786	19,222
Public Service Company of New Hampshire | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	10,300	7,000	4,900
Public Service Company of New Hampshire | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	1,600	4,200	3,400
Public Service Company of New Hampshire | Equity in Earnings of Unconsolidated Affiliates
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	100	100	300
Public Service Company of New Hampshire | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	2,400	2,400	10,500
Public Service Company of New Hampshire | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 200	$ 100	$ 100