FAIR VALUE OF FINANCIAL INSTRUMENTS - SCHEDULE OF FINANCIAL ASSETS AND LIABILITIES MEASURED AT FAIR VALUE ON A RECURRING BASIS (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Total financial assets	$ 5,609,878	$ 1,966,733
Liabilities
Total financial liabilities	1,531,017	803,178
Equity securities - user-held fractional shares
Assets
Other current assets	1,530,971	802,483
Equity securities - securities owned
Assets
Other current assets	22,769	3,222
U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		134,994
Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	46	695
Equity securities - repurchase obligations
Liabilities
Other current liabilities	1,530,971	802,483
Money market funds
Assets
Cash equivalents	4,006,138	1,026,034
U.S. Treasury securities
Assets
Cash equivalents	50,000
Level 1
Assets
Total financial assets	5,609,878	1,966,733
Liabilities
Total financial liabilities	1,531,017	803,178
Level 1 | Equity securities - user-held fractional shares
Assets
Other current assets	1,530,971	802,483
Level 1 | Equity securities - securities owned
Assets
Other current assets	22,769	3,222
Level 1 | U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		134,994
Level 1 | Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	46	695
Level 1 | Equity securities - repurchase obligations
Liabilities
Other current liabilities	1,530,971	802,483
Level 1 | Money market funds
Assets
Cash equivalents	4,006,138	1,026,034
Level 1 | U.S. Treasury securities
Assets
Cash equivalents	50,000
Level 2
Assets
Total financial assets	0	0
Liabilities
Total financial liabilities	0	0
Level 2 | Equity securities - user-held fractional shares
Assets
Other current assets	0	0
Level 2 | Equity securities - securities owned
Assets
Other current assets	0	0
Level 2 | U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		0
Level 2 | Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	0	0
Level 2 | Equity securities - repurchase obligations
Liabilities
Other current liabilities	0	0
Level 2 | Money market funds
Assets
Cash equivalents	0	0
Level 2 | U.S. Treasury securities
Assets
Cash equivalents	0
Level 3
Assets
Total financial assets	0	0
Liabilities
Total financial liabilities	0	0
Level 3 | Equity securities - user-held fractional shares
Assets
Other current assets	0	0
Level 3 | Equity securities - securities owned
Assets
Other current assets	0	0
Level 3 | U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		0
Level 3 | Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	0	0
Level 3 | Equity securities - repurchase obligations
Liabilities
Other current liabilities	0	0
Level 3 | Money market funds
Assets
Cash equivalents	0	$ 0
Level 3 | U.S. Treasury securities
Assets
Cash equivalents	$ 0