SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 729,993	$ 15,432
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	829,003	117,063	$ 151,880	$ 209,324
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	51,300	52,500
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	45,600	46,000
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	2,100	3,200
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	8,173	0
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	17,235	4,971	14,281	91,613
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,600	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 and Cash Equivalents as reported on the Balance Sheets	87	52
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	12,409	6,312	6,508	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 and Cash Equivalents as reported on the Balance Sheets	767	413
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	22,595	36,688	$ 24,012	$ 52,723
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	19,100	32,500
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	$ 2,100	$ 3,200