SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 33,381	$ 53,873
Cash and Restricted Cash as reported on the Statements of Cash Flows	132,633	166,418	$ 177,992	$ 521,752
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	66,400	81,500
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,300	13,700
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,500	17,300
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	0	10,213
Cash and Restricted Cash as reported on the Statements of Cash Flows	1,068	12,243	24,128	20,327
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	1,100	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 as reported on the Balance Sheets	2,730	6,740
Cash and Restricted Cash as reported on the Statements of Cash Flows	4,092	22,785	9,135	345,293
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	1,400	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 as reported on the Balance Sheets	252	240
Cash and Restricted Cash as reported on the Statements of Cash Flows	34,081	35,004	$ 37,759	$ 36,812
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	30,600	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	$ 3,200	$ 3,200