Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		$ 799,719
Securities purchased under agreements to resell	$ 0	0
Due from Employees	46,161	43,670
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		665,943
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		133,776
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		0
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Estimate Not Practicable, Cash	35,424	79,550
Deposits with clearing organization	59,352	23,297
Securities borrowed	110,932	99,635
Receivables from brokers	30,133	19,024
Securities failed to deliver	17,840	7,173
Clearing organizations	28,955	36,269
Other	15,622	1,316
Total Receivable from brokers, dealers and clearing organizations	203,482	163,417
Receivable from customers	1,110,835	796,934
Due from Employees	46,161	43,670
Fair Value, Estimate Not Practicable, Investment	85,552	73,971
Securities loaned	249,499	234,343
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	4,102	4,548
Securities failed to receive	6,218	14,603
Other	70	267,214
Total payables to brokers, dealers and clearing organizations	259,889	520,708
Payables to customers	502,807	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	342,438	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	127,033	154,988
Fair Value Estimate Not Practicable Bank Call Loans	82,000
Fair Value, Nonrecurring [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Estimate Not Practicable, Cash	35,424	79,550
Deposits with clearing organization	59,352	23,297
Fair Value, Nonrecurring [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities borrowed	110,932	99,635
Receivables from brokers	30,133	19,024
Securities failed to deliver	17,840	7,173
Clearing organizations	28,955	36,269
Other	15,622	1,316
Total Receivable from brokers, dealers and clearing organizations	203,482	163,417
Receivable from customers	1,110,835	796,934
Due from Employees	46,161	43,670
Fair Value, Estimate Not Practicable, Investment	85,552	73,971
Securities loaned	249,499	234,343
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	4,102	4,548
Securities failed to receive	6,218	14,603
Other	70	267,214
Total payables to brokers, dealers and clearing organizations	259,889	520,708
Payables to customers	502,807	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	342,438	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	127,033	154,988
Fair Value Estimate Not Practicable Bank Call Loans	82,000
Fair Value, Nonrecurring [Member] | Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Estimate Not Practicable, Cash	35,424	79,550
Deposits with clearing organization	59,352	23,297
Securities borrowed	110,932	99,635
Receivables from brokers	30,133	19,024
Securities failed to deliver	17,840	7,173
Clearing organizations	28,955	36,269
Other	15,622	1,316
Total Receivable from brokers, dealers and clearing organizations	203,482	163,417
Receivable from customers	1,110,835	796,934
Due from Employees	46,161	43,670
Fair Value, Estimate Not Practicable, Investment	85,552	73,971
Securities loaned	249,499	234,343
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	4,102	4,548
Securities failed to receive	6,218	14,603
Other	70	267,214
Total payables to brokers, dealers and clearing organizations	259,889	520,708
Payables to customers	502,807	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	342,438	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	125,000	$ 150,000
Fair Value Estimate Not Practicable Bank Call Loans	$ 82,000