SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 34,112	$ 106,599
Cash and Restricted Cash as reported on the Statements of Cash Flows	175,604	264,950	$ 133,689	$ 117,063
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	58,400	73,600
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	38,400	41,200
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	44,700	43,600
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	9,256	90,801
Cash and Restricted Cash as reported on the Statements of Cash Flows	18,288	99,809	8,767	4,971
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,600	8,700
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	400	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	57	102
Cash and Restricted Cash as reported on the Statements of Cash Flows	17,373	17,410	9,129	6,312
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,200	17,200
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	1,988	141
Cash and Restricted Cash as reported on the Statements of Cash Flows	27,988	39,555	$ 20,390	$ 36,688
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	22,200	36,800
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	600	600
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 2,100