Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
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		$ 837,584
Securities purchased under agreements to resell	$ 0	290
Due from Employees	43,670	44,058
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		686,242
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		129,643
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		21,699
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Estimate Not Practicable, Cash	79,550	80,175
Deposits with clearing organization	23,297	33,079
Securities borrowed	99,635	108,144
Receivables from brokers	19,024	20,140
Securities failed to deliver	7,173	7,021
Clearing organizations	36,269	28,777
Other	1,316	2,411
Total Receivable from brokers, dealers and clearing organizations	163,417	166,493
Receivable from customers	796,934	720,777
Securities purchased under agreements to resell		290
Due from Employees	43,670	44,058
Fair Value, Estimate Not Practicable, Investment	73,971	59,765
Drafts payable		16,348
Bank call loans		15,000
Securities loaned	234,343	146,815
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	4,548	158
Securities failed to receive	14,603	27,799
Other	267,214	113,628
Total payables to brokers, dealers and clearing organizations	520,708	288,400
Payables to customers	334,735	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	287,265	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	154,988	199,722
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	79,550	80,175
Deposits with clearing organization	23,297	33,079
Drafts payable		16,348
Fair Value, Nonrecurring [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities borrowed	99,635	108,144
Receivables from brokers	19,024	20,140
Securities failed to deliver	7,173	7,021
Clearing organizations	36,269	28,777
Other	1,316	2,411
Total Receivable from brokers, dealers and clearing organizations	163,417	166,493
Receivable from customers	796,934	720,777
Due from Employees	43,670	44,058
Fair Value, Estimate Not Practicable, Investment	73,971	59,765
Bank call loans		15,000
Securities loaned	234,343	146,815
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	4,548	158
Securities failed to receive	14,603	27,799
Other	267,214	113,628
Total payables to brokers, dealers and clearing organizations	520,708	288,400
Payables to customers	334,735	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	287,265	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	154,988	199,722
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	79,550	80,175
Deposits with clearing organization	23,297	33,079
Securities borrowed	99,635	108,144
Receivables from brokers	19,024	20,140
Securities failed to deliver	7,173	7,021
Clearing organizations	36,269	28,777
Other	1,316	2,411
Total Receivable from brokers, dealers and clearing organizations	163,417	166,493
Receivable from customers	796,934	720,777
Securities purchased under agreements to resell		290
Due from Employees	43,670	44,058
Fair Value, Estimate Not Practicable, Investment	73,971	59,765
Drafts payable		16,348
Bank call loans		15,000
Securities loaned	234,343	146,815
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	4,548	158
Securities failed to receive	14,603	27,799
Other	267,214	113,628
Total payables to brokers, dealers and clearing organizations	520,708	288,400
Payables to customers	334,735	336,616
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	287,265	484,218
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 150,000	$ 200,000