Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell	$ 39	$ 290
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	995,370	837,584
Fair Value, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,791	2,411
Receivable from customers	807,581	720,777
Fair Value, Estimate Not Practicable, Investment	69,070	59,765
Drafts payable	17,456	16,348
Clearing organizations	25,604	28,777
Securities loaned	247,714	146,815
Fair Value, Estimate not Practicable, Due from Employees	43,413	44,058
Total Receivable from brokers, dealers and clearing organizations	167,962	166,493
Securities failed to receive	30,570	27,799
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	8,667	158
Bank call loans	50,100	15,000
Securities purchased under agreements to resell	39	290
Receivables from brokers	22,521	20,140
Securities failed to deliver	14,898	7,021
Securities borrowed	103,148	108,144
Deposits with clearing organization	34,198	33,079
Other	129,987	113,628
Total payables to brokers, dealers and clearing organizations	416,938	288,400
Payables to customers	325,754	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	521,837	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	205,356	199,722
Fair Value, Estimate Not Practicable, Cash	60,807	80,175
Fair Value, Nonrecurring [Member] | Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,791	2,411
Receivable from customers	807,581	720,777
Fair Value, Estimate Not Practicable, Investment	69,070	59,765
Drafts payable	17,456	16,348
Clearing organizations	25,604	28,777
Securities loaned	247,714	146,815
Fair Value, Estimate not Practicable, Due from Employees	43,413	44,058
Total Receivable from brokers, dealers and clearing organizations	167,962	166,493
Securities failed to receive	30,570	27,799
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	8,667	158
Bank call loans	50,100	15,000
Securities purchased under agreements to resell	39	290
Receivables from brokers	22,521	20,140
Securities failed to deliver	14,898	7,021
Securities borrowed	103,148	108,144
Deposits with clearing organization	34,198	33,079
Other	129,987	113,628
Total payables to brokers, dealers and clearing organizations	416,938	288,400
Payables to customers	325,754	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	521,837	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	200,000	200,000
Fair Value, Estimate Not Practicable, Cash	60,807	80,175
Fair Value, Nonrecurring [Member] | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	17,456	16,348
Deposits with clearing organization	34,198	33,079
Fair Value, Estimate Not Practicable, Cash	60,807	80,175
Fair Value, Nonrecurring [Member] | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,791	2,411
Receivable from customers	807,581	720,777
Fair Value, Estimate Not Practicable, Investment	69,070	59,765
Clearing organizations	25,604	28,777
Securities loaned	247,714	146,815
Fair Value, Estimate not Practicable, Due from Employees	43,413	44,058
Total Receivable from brokers, dealers and clearing organizations	167,962	166,493
Securities failed to receive	30,570	27,799
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	8,667	158
Bank call loans	50,100	15,000
Securities purchased under agreements to resell	39	290
Receivables from brokers	22,521	20,140
Securities failed to deliver	14,898	7,021
Securities borrowed	103,148	108,144
Other	129,987	113,628
Total payables to brokers, dealers and clearing organizations	416,938	288,400
Payables to customers	325,754	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	521,837	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 205,356	$ 199,722