SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 42,200	$ 374,600
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	177,992	521,752	$ 156,676	$ 221,008
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	75,600	102,200
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	42,600	25,400
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,600	19,600
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	14,200	11,300
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	24,128	20,327	17,911	74,788
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	9,000	8,800
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	900	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	6,500	327,700
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	9,135	345,293	22,724	18,179
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	2,300	17,500
NSTAR Electric | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	300	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	37,759	36,812	$ 34,299	$ 35,126
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	32,900	33,100
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	1,600	400
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200