SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 64,890	$ 15,432
Cash and Restricted Cash reported on the Statements of Cash Flows	176,920	117,063	$ 167,163	$ 209,324
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	63,400	52,500
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	45,400	46,000
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	3,200	3,200
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	6,918	0
Cash and Restricted Cash reported on the Statements of Cash Flows	15,998	4,971	7,094	91,613
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,700	4,600
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	400	400
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	1,158	52
Cash and Restricted Cash reported on the Statements of Cash Flows	13,482	6,312	6,366	14,659
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	12,200	6,200
NSTAR Electric | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	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	77	413
Cash and Restricted Cash reported on the Statements of Cash Flows	34,870	36,688	$ 39,087	$ 52,723
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	30,900	32,500
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	700	600
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200