SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 485,700	$ 66,800
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	602,256	221,008	$ 218,129	$ 264,950
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	76,500	78,200
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	20,800	31,300
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	19,300	44,700
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	14,300	55,800
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	23,120	74,788	71,762	99,809
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,700	18,700
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	300
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	453,600	700
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	470,945	18,179	20,838	17,410
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,300	17,400
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	2,200	0
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	24,246	35,126	$ 26,173	$ 39,555
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	18,700	31,400
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	500
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200