SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash and Restricted Cash Balance as Reported (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	$ 135,400	$ 26,700
Cash and Restricted Cash as reported on the Statements of Cash Flows	246,245	127,308	$ 166,418	$ 521,752
CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	87,600	1,100
Cash and Restricted Cash as reported on the Statements of Cash Flows	88,676	2,109	12,243	20,327
NSTAR Electric
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	8,300	900
Cash and Restricted Cash as reported on the Statements of Cash Flows	25,944	9,023	22,785	345,293
PSNH
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	13,700	1,400
Cash and Restricted Cash as reported on the Statements of Cash Flows	55,186	37,243	$ 35,004	$ 36,812
Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	93,600	75,800
Special Deposits | CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	1,100	1,000
Special Deposits | NSTAR Electric
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	17,600	8,100
Special Deposits | PSNH
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	38,300	32,700
Assets Held for Sale, Current
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	5,800
Assets Held for Sale, Current | CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Assets Held for Sale, Current | NSTAR Electric
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Assets Held for Sale, Current | PSNH
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Marketable Securities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	14,000	10,000
Marketable Securities | CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Marketable Securities | NSTAR Electric
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Marketable Securities | PSNH
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Other Long-Term Assets
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	3,200	9,000
Other Long-Term Assets | CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Other Long-Term Assets | NSTAR Electric
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
Other Long-Term Assets | PSNH
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	$ 3,200	$ 3,100