Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - Fair Value, Nonrecurring - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	$ 47,453	$ 213,759
Restricted cash	127,781	127,765
Deposits with clearing organization	62,805	37,885
Securities borrowed	98,276	99,752
Receivables from brokers	38,165	39,716
Securities failed to deliver	37,028	9,212
Clearing organizations	19,758	19,518
Other	18,062	1,693
Total Receivable from brokers, dealers and clearing organizations	211,289	169,891
Receivable from customers	1,261,305	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	57,879	53,983
Investments	93,055	99,169
Bank call loans	78,150	69,500
Securities loaned	300,323	244,223
Payable to brokers	566	2,077
Securities failed to receive	18,614	6,457
Total payables to brokers, dealers and clearing organizations	319,503	421,770
Other		169,013
Payables to customers	506,988	456,958
Securities sold under agreements to repurchase	481,151	277,322
Senior secured notes	125,054	131,094
Reported Value Measurement
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	47,453	213,759
Restricted cash	127,781	127,765
Deposits with clearing organization	62,805	37,885
Securities borrowed	98,276	99,752
Receivables from brokers	38,165	39,716
Securities failed to deliver	37,028	9,212
Clearing organizations	19,758	19,518
Other	18,062	1,693
Total Receivable from brokers, dealers and clearing organizations	211,289	169,891
Receivable from customers	1,261,305	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	57,879	53,983
Investments	93,055	99,169
Bank call loans	78,150	69,500
Securities loaned	300,323	244,223
Payable to brokers	566	2,077
Securities failed to receive	18,614	6,457
Total payables to brokers, dealers and clearing organizations	319,503	421,770
Other		169,013
Payables to customers	506,988	456,958
Securities sold under agreements to repurchase	481,151	277,322
Senior secured notes	125,000	125,000
Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	47,453	213,759
Restricted cash	127,781	127,765
Deposits with clearing organization	62,805	37,885
Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities borrowed	98,276	99,752
Receivables from brokers	38,165	39,716
Securities failed to deliver	37,028	9,212
Clearing organizations	19,758	19,518
Other	18,062	1,693
Total Receivable from brokers, dealers and clearing organizations	211,289	169,891
Receivable from customers	1,261,305	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	57,879	53,983
Investments	93,055	99,169
Bank call loans	78,150	69,500
Securities loaned	300,323	244,223
Payable to brokers	566	2,077
Securities failed to receive	18,614	6,457
Total payables to brokers, dealers and clearing organizations	319,503	421,770
Other		169,013
Payables to customers	506,988	456,958
Securities sold under agreements to repurchase	481,151	277,322
Senior secured notes	$ 125,054	$ 131,094