SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Summary of Accounting Policies [Line Items]
Other income, net	$ 88,981	$ 71,561
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	34,800	54,300
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	15,500	9,900
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	3,800	400
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(1,700)	(200)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	23,100	6,700
Other
Summary of Accounting Policies [Line Items]
Other income, net	13,500	500
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	14,937	19,564
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	9,500	15,900
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	4,100	2,800
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
CL&P | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(600)	(400)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,900	1,300
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	39,873	29,231
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	14,700	21,000
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	9,500	4,900
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100	0
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(500)	(300)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	16,000	3,500
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	100
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	5,717	7,509
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	4,200	6,600
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	700	400
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
PSNH | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(100)	200
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	900	300
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0