SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 259,213	$ 53,873
Cash and Restricted Cash as reported on the Statements of Cash Flows	360,396	166,418	$ 162,767	$ 521,752
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	68,700	81,500
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,000	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	29,006	10,213
Cash and Restricted Cash as reported on the Statements of Cash Flows	31,055	12,243	18,409	20,327
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	2,100	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	20,226	6,740
Cash and Restricted Cash as reported on the Statements of Cash Flows	36,503	22,785	18,355	345,293
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,300	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	14,067	240
Cash and Restricted Cash as reported on the Statements of Cash Flows	34,311	35,004	$ 23,012	$ 36,812
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,000	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