OTHER CURRENT ASSETS - DETAIL OF OTHER CURRENT ASSETS (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
User-held fractional shares	$ 1,530,971	$ 802,483
Prepaid expenses	98,303	28,629
Securities owned	22,769	3,222
Restricted cash	1,690	0
Other	20,280	16,804
Other current assets	$ 1,674,013	$ 851,138