SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 78,800	$ 374,600
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	180,907	521,752	$ 602,256	$ 221,008
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	68,400	102,200
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,200	25,400
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,500	19,600
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	9,800	11,300
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	18,943	20,327	23,120	74,788
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	9,100	8,800
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	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	6,000	327,700
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	14,846	345,293	470,945	18,179
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,800	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	33,900	100
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	55,307	36,812	$ 24,246	$ 35,126
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	18,200	33,100
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	400
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200