Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	$ 544,062	$ 610,517
Fair Value, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,290	15,622
Receivable from customers	1,225,140	1,110,835
Fair Value, Estimate Not Practicable, Investment	93,384	85,552
Drafts payable	17,133
Clearing organizations	31,525	28,955
Securities loaned	267,521	249,499
Fair Value, Estimate not Practicable, Due from Employees	52,119	46,161
Total Receivable from brokers, dealers and clearing organizations	235,443	203,482
Securities failed to receive	13,599	6,218
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,599	4,102
Bank call loans	79,600	82,000
Receivables from brokers	35,210	30,133
Securities failed to deliver	48,272	17,840
Securities borrowed	119,146	110,932
Deposits with clearing organization	51,770	59,352
Other	65,416	70
Total payables to brokers, dealers and clearing organizations	349,135	259,889
Payables to customers	462,178	502,807
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	231,601	342,438
Fair Value, Estimate not Practicable, Senior Secured Notes	130,920	127,033
Fair Value, Estimate Not Practicable, Cash	39,798	35,424
Fair Value, Nonrecurring [Member] \| Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,290	15,622
Receivable from customers	1,225,140	1,110,835
Fair Value, Estimate Not Practicable, Investment	93,384	85,552
Drafts payable	17,133
Clearing organizations	31,525	28,955
Securities loaned	267,521	249,499
Fair Value, Estimate not Practicable, Due from Employees	52,119	46,161
Total Receivable from brokers, dealers and clearing organizations	235,443	203,482
Securities failed to receive	13,599	6,218
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,599	4,102
Bank call loans	79,600	82,000
Receivables from brokers	35,210	30,133
Securities failed to deliver	48,272	17,840
Securities borrowed	119,146	110,932
Deposits with clearing organization	51,770	59,352
Other	65,416	70
Total payables to brokers, dealers and clearing organizations	349,135	259,889
Payables to customers	462,178	502,807
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	231,601	342,438
Fair Value, Estimate not Practicable, Senior Secured Notes	125,000	125,000
Fair Value, Estimate Not Practicable, Cash	39,798	35,424
Fair Value, Nonrecurring [Member] \| Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	17,133
Deposits with clearing organization	51,770	59,352
Fair Value, Estimate Not Practicable, Cash	39,798	35,424
Fair Value, Nonrecurring [Member] \| Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,290	15,622
Receivable from customers	1,225,140	1,110,835
Fair Value, Estimate Not Practicable, Investment	93,384	85,552
Clearing organizations	31,525	28,955
Securities loaned	267,521	249,499
Fair Value, Estimate not Practicable, Due from Employees	52,119	46,161
Total Receivable from brokers, dealers and clearing organizations	235,443	203,482
Securities failed to receive	13,599	6,218
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,599	4,102
Bank call loans	79,600	82,000
Receivables from brokers	35,210	30,133
Securities failed to deliver	48,272	17,840
Securities borrowed	119,146	110,932
Other	65,416	70
Total payables to brokers, dealers and clearing organizations	349,135	259,889
Payables to customers	462,178	502,807
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	231,601	342,438
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 130,920	$ 127,033

This item represents a financial instrument (as defined) for which it is not practicable to estimate fair value. Such disclosure may be for an individual financial instrument or a class of financial instruments.