SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 46,175	$ 66,773
Cash and Restricted Cash as reported on the Statements of Cash Flows	164,589	221,008	$ 175,604	$ 264,950
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	73,300	78,200
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	26,000	31,300
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	19,100	44,700
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	21,628	55,804
Cash and Restricted Cash as reported on the Statements of Cash Flows	30,584	74,788	18,288	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	300	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 as reported on the Balance Sheets	721	745
Cash and Restricted Cash as reported on the Statements of Cash Flows	18,144	18,179	17,373	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	100	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 as reported on the Balance Sheets	3,827	15
Cash and Restricted Cash as reported on the Statements of Cash Flows	24,058	35,126	$ 27,988	$ 39,555
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	16,600	31,400
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	500	500
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200