SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 374,600	$ 66,800
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	521,752	221,008	$ 264,950	$ 117,063
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	11,300	55,800
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	20,327	74,788	99,809	4,971
NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	327,700	700
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	345,293	18,179	17,410	6,312
PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	100	0
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	36,812	35,126	$ 39,555	$ 36,688
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	102,200	78,200
Special Deposits | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,800	18,700
Special Deposits | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,500	17,400
Special Deposits | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	33,100	31,400
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	25,400	31,300
Marketable Securities | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	200	300
Marketable Securities | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	100
Marketable Securities | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	400	500
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	19,600	44,700
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