INVESTMENTS AND FAIR VALUE MEASUREMENT - Schedule of Financial Asset and Liabilities Measured At Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Assets
User-held fractional shares	$ 1,834,479	$ 802,483
Liabilities
Fractional shares repurchase obligation	1,834,479	802,483
Fair Value, Recurring
Assets
User-held fractional shares	1,834,479	802,483
Total financial assets	5,878,831	1,966,733
Liabilities
Fractional shares repurchase obligation	1,834,479	802,483
Total financial liabilities	1,834,576	803,178
Fair Value, Recurring | U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		134,994
Fair Value, Recurring | Asset-backed securities
Assets
Investments	5,078
Fair Value, Recurring | Commercial paper
Assets
Investments	13,717
Fair Value, Recurring | Corporate bonds
Assets
Investments	7,384
Fair Value, Recurring | Government bonds
Assets
Investments	1,010
Fair Value, Recurring | Equity securities - securities owned
Assets
Other current assets	13,611	3,222
Fair Value, Recurring | Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	97	695
Fair Value, Recurring | Money market funds
Assets
Cash equivalents	4,003,552	1,026,034
Fair Value, Recurring | Level 1
Assets
User-held fractional shares	1,834,479	802,483
Total financial assets	5,852,652	1,966,733
Liabilities
Fractional shares repurchase obligation	1,834,479	802,483
Total financial liabilities	1,834,576	803,178
Fair Value, Recurring | Level 1 | U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		134,994
Fair Value, Recurring | Level 1 | Asset-backed securities
Assets
Investments	0
Fair Value, Recurring | Level 1 | Commercial paper
Assets
Investments	0
Fair Value, Recurring | Level 1 | Corporate bonds
Assets
Investments	0
Fair Value, Recurring | Level 1 | Government bonds
Assets
Investments	1,010
Fair Value, Recurring | Level 1 | Equity securities - securities owned
Assets
Other current assets	13,611	3,222
Fair Value, Recurring | Level 1 | Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	97	695
Fair Value, Recurring | Level 1 | Money market funds
Assets
Cash equivalents	4,003,552	1,026,034
Fair Value, Recurring | Level 2
Assets
User-held fractional shares	0	0
Total financial assets	26,179	0
Liabilities
Fractional shares repurchase obligation	0	0
Total financial liabilities	0	0
Fair Value, Recurring | Level 2 | U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		0
Fair Value, Recurring | Level 2 | Asset-backed securities
Assets
Investments	5,078
Fair Value, Recurring | Level 2 | Commercial paper
Assets
Investments	13,717
Fair Value, Recurring | Level 2 | Corporate bonds
Assets
Investments	7,384
Fair Value, Recurring | Level 2 | Government bonds
Assets
Investments	0
Fair Value, Recurring | Level 2 | Equity securities - securities owned
Assets
Other current assets	0	0
Fair Value, Recurring | Level 2 | Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	0	0
Fair Value, Recurring | Level 2 | Money market funds
Assets
Cash equivalents	0	0
Fair Value, Recurring | Level 3
Assets
User-held fractional shares	0	0
Total financial assets	0	0
Liabilities
Fractional shares repurchase obligation	0	0
Total financial liabilities	0	0
Fair Value, Recurring | Level 3 | U.S. Treasury securities
Assets
Cash and securities segregated under federal and other regulations		0
Fair Value, Recurring | Level 3 | Asset-backed securities
Assets
Investments	0
Fair Value, Recurring | Level 3 | Commercial paper
Assets
Investments	0
Fair Value, Recurring | Level 3 | Corporate bonds
Assets
Investments	0
Fair Value, Recurring | Level 3 | Government bonds
Assets
Investments	0
Fair Value, Recurring | Level 3 | Equity securities - securities owned
Assets
Other current assets	0	0
Fair Value, Recurring | Level 3 | Equity securities - referral program liability
Liabilities
Accounts payable and accrued expenses	0	0
Fair Value, Recurring | Level 3 | Money market funds
Assets
Cash equivalents	$ 0	$ 0