SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 53,900	$ 374,600
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	166,418	521,752	$ 221,008	$ 264,950
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	10,200	11,300
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	12,243	20,327	74,788	99,809
NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	6,700	327,700
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	22,785	345,293	18,179	17,410
PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	200	100
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	35,004	36,812	$ 35,126	$ 39,555
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	81,500	102,200
Special Deposits | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	2,000	8,800
Special Deposits | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,100	17,500
Special Deposits | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	31,600	33,100
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	13,700	25,400
Marketable Securities | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	200
Marketable Securities | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	100
Marketable Securities | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	400
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,300	19,600
Other Long-Term Assets | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
Other Long-Term Assets | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
Other Long-Term Assets | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200