SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 97,900	$ 53,900
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	202,801	166,418	$ 180,907	$ 521,752
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	66,500	81,500
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	21,500	13,700
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,900	17,300
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	10,116	10,213
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	11,117	12,243	18,943	20,327
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	1,000	2,000
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
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	4,467	6,740
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	15,968	22,785	14,846	345,293
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	11,500	16,100
NSTAR Electric | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
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	4,673	240
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	26,693	35,004	$ 55,307	$ 36,812
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	18,900	31,600
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,100	$ 3,200