SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 108,590	$ 132,777	$ 128,366
Unrealized gain associated with investment	14,200	42,200	3,800
Renewable Energy Fund
Summary of Accounting Policies [Line Items]
Other-than-temporary impairment			2,800
Access Northeast Project
Summary of Accounting Policies [Line Items]
Other-than-temporary impairment			32,900
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	44,400	31,300	60,800
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	42,000	45,000	44,000
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	14,200	42,200	3,800
Unrealized gain associated with investment	2,400	20,400	17,600
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	1,100	800	(4,000)
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	4,800	12,800	18,100
Gains on Sales of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	1,800	300	5,100
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	300	400	600
The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net	20,774	17,531	22,663
The Connecticut Light and Power Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	3,800	500	9,500
The Connecticut Light and Power Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	13,800	13,200	12,200
The Connecticut Light and Power Company | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	100	100
The Connecticut Light and Power Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	1,100	2,300	(3,000)
The Connecticut Light and Power Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	2,000	1,500	3,700
The Connecticut Light and Power Company | Gains on Sales 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	100	(100)	200
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net	52,017	44,577	53,066
NSTAR Electric Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	29,300	23,500	36,000
NSTAR Electric Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	21,500	19,800	15,600
NSTAR Electric Company | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	400	700	700
NSTAR Electric Company | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(800)	(400)	(500)
NSTAR Electric Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	900	700	800
NSTAR Electric Company | Gains on Sales of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	300	100	500
NSTAR Electric Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	400	200	0
Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net	13,786	19,222	27,672
Public Service Company of New Hampshire | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	7,000	4,900	9,900
Public Service Company of New Hampshire | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	4,200	3,400	0
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	300	(800)
Public Service Company of New Hampshire | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	2,400	10,500	14,100
Public Service Company of New Hampshire | Gains on Sales of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	4,400
Public Service Company of New Hampshire | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 100	$ 100	$ 100