Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 605,330	$ 610,517
Fair Value, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	717	15,622
Receivable from customers	1,152,499	1,110,835
Fair Value, Estimate Not Practicable, Investment	88,963	85,552
Drafts payable	12,392
Clearing organizations	28,221	28,955
Securities loaned	257,342	249,499
Fair Value, Estimate not Practicable, Due from Employees	48,847	46,161
Total Receivable from brokers, dealers and clearing organizations	233,888	203,482
Securities failed to receive	38,100	6,218
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	1,336	4,102
Bank call loans	75,100	82,000
Receivables from brokers	32,994	30,133
Securities failed to deliver	62,193	17,840
Securities borrowed	109,763	110,932
Deposits with clearing organization	51,395	59,352
Other	124,339	70
Total payables to brokers, dealers and clearing organizations	421,117	259,889
Payables to customers	462,450	502,807
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	7,150	342,438
Fair Value, Estimate not Practicable, Senior Secured Notes	129,375	127,033
Fair Value, Estimate Not Practicable, Cash	28,545	35,424
Fair Value, Nonrecurring [Member] \| Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	717	15,622
Receivable from customers	1,152,499	1,110,835
Fair Value, Estimate Not Practicable, Investment	88,963	85,552
Drafts payable	12,392
Clearing organizations	28,221	28,955
Securities loaned	257,342	249,499
Fair Value, Estimate not Practicable, Due from Employees	48,847	46,161
Total Receivable from brokers, dealers and clearing organizations	233,888	203,482
Securities failed to receive	38,100	6,218
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	1,336	4,102
Bank call loans	75,100	82,000
Receivables from brokers	32,994	30,133
Securities failed to deliver	62,193	17,840
Securities borrowed	109,763	110,932
Deposits with clearing organization	51,395	59,352
Other	124,339	70
Total payables to brokers, dealers and clearing organizations	421,117	259,889
Payables to customers	462,450	502,807
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	7,150	342,438
Fair Value, Estimate not Practicable, Senior Secured Notes	125,000	125,000
Fair Value, Estimate Not Practicable, Cash	28,545	35,424
Fair Value, Nonrecurring [Member] \| Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	12,392
Deposits with clearing organization	51,395	59,352
Fair Value, Estimate Not Practicable, Cash	28,545	35,424
Fair Value, Nonrecurring [Member] \| Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	717	15,622
Receivable from customers	1,152,499	1,110,835
Fair Value, Estimate Not Practicable, Investment	88,963	85,552
Clearing organizations	28,221	28,955
Securities loaned	257,342	249,499
Fair Value, Estimate not Practicable, Due from Employees	48,847	46,161
Total Receivable from brokers, dealers and clearing organizations	233,888	203,482
Securities failed to receive	38,100	6,218
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	1,336	4,102
Bank call loans	75,100	82,000
Receivables from brokers	32,994	30,133
Securities failed to deliver	62,193	17,840
Securities borrowed	109,763	110,932
Other	124,339	70
Total payables to brokers, dealers and clearing organizations	421,117	259,889
Payables to customers	462,450	502,807
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	7,150	342,438
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 129,375	$ 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.