SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 36,000	$ 374,600
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	162,767	521,752	$ 164,589	$ 221,008
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	89,000	102,200
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,900	25,400
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	19,900	19,600
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	9,300	11,300
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	18,409	20,327	30,584	74,788
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,900	8,800
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	200	200
CL&P | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	600	327,700
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	18,355	345,293	18,144	18,179
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,800	17,500
NSTAR Electric | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	100
NSTAR Electric | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	100	100
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	23,012	36,812	$ 24,058	$ 35,126
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	19,500	33,100
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	200	400
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200