Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell	$ 0	$ 290
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	653,194	837,584
Fair Value, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	2,017	2,411
Receivable from customers	754,927	720,777
Fair Value, Estimate Not Practicable, Investment	68,919	59,765
Drafts payable	17,794	16,348
Clearing organizations	24,775	28,777
Securities loaned	209,298	146,815
Fair Value, Estimate not Practicable, Due from Employees	43,920	44,058
Total Receivable from brokers, dealers and clearing organizations	157,731	166,493
Securities failed to receive	13,228	27,799
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	7,040	158
Bank call loans		15,000
Securities purchased under agreements to resell		290
Receivables from brokers	13,839	20,140
Securities failed to deliver	13,362	7,021
Securities borrowed	103,738	108,144
Deposits with clearing organization	37,269	33,079
Other	134,707	113,628
Total payables to brokers, dealers and clearing organizations	364,273	288,400
Payables to customers	313,056	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	253,720	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	154,907	199,722
Fair Value, Estimate Not Practicable, Cash	26,193	80,175
Fair Value, Nonrecurring [Member] | Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	2,017	2,411
Receivable from customers	754,927	720,777
Fair Value, Estimate Not Practicable, Investment	68,919	59,765
Drafts payable	17,794	16,348
Clearing organizations	24,775	28,777
Securities loaned	209,298	146,815
Fair Value, Estimate not Practicable, Due from Employees	43,920	44,058
Total Receivable from brokers, dealers and clearing organizations	157,731	166,493
Securities failed to receive	13,228	27,799
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	7,040	158
Bank call loans		15,000
Securities purchased under agreements to resell		290
Receivables from brokers	13,839	20,140
Securities failed to deliver	13,362	7,021
Securities borrowed	103,738	108,144
Deposits with clearing organization	37,269	33,079
Other	134,707	113,628
Total payables to brokers, dealers and clearing organizations	364,273	288,400
Payables to customers	313,056	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	253,720	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	150,000	200,000
Fair Value, Estimate Not Practicable, Cash	26,193	80,175
Fair Value, Nonrecurring [Member] | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	17,794	16,348
Deposits with clearing organization	37,269	33,079
Fair Value, Estimate Not Practicable, Cash	26,193	80,175
Fair Value, Nonrecurring [Member] | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	2,017	2,411
Receivable from customers	754,927	720,777
Fair Value, Estimate Not Practicable, Investment	68,919	59,765
Clearing organizations	24,775	28,777
Securities loaned	209,298	146,815
Fair Value, Estimate not Practicable, Due from Employees	43,920	44,058
Total Receivable from brokers, dealers and clearing organizations	157,731	166,493
Securities failed to receive	13,228	27,799
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	7,040	158
Bank call loans		15,000
Securities purchased under agreements to resell		290
Receivables from brokers	13,839	20,140
Securities failed to deliver	13,362	7,021
Securities borrowed	103,738	108,144
Other	134,707	113,628
Total payables to brokers, dealers and clearing organizations	364,273	288,400
Payables to customers	313,056	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	253,720	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 154,907	$ 199,722