Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
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	$ 309,346	$ 799,719
Fair Value, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	2,833	1,316
Receivable from customers	964,772	796,934
Fair Value, Estimate Not Practicable, Investment	60,631	73,971
Drafts payable	19,066
Clearing organizations	28,463	36,269
Securities loaned	181,745	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,960	43,670
Total Receivable from brokers, dealers and clearing organizations	198,422	163,417
Securities failed to receive	45,593	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	5,079	4,548
Bank call loans	203,100
Receivables from brokers	37,588	19,024
Securities failed to deliver	26,694	7,173
Securities borrowed	102,844	99,635
Deposits with clearing organization	82,987	23,297
Other	6,005	267,214
Total payables to brokers, dealers and clearing organizations	238,422	520,708
Payables to customers	413,646	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	124,103	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	140,624	154,988
Fair Value, Estimate Not Practicable, Cash	23,540	79,550
Fair Value, Nonrecurring [Member] | Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	2,833	1,316
Receivable from customers	964,772	796,934
Fair Value, Estimate Not Practicable, Investment	60,631	73,971
Drafts payable	19,066
Clearing organizations	28,463	36,269
Securities loaned	181,745	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,960	43,670
Total Receivable from brokers, dealers and clearing organizations	198,422	163,417
Securities failed to receive	45,593	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	5,079	4,548
Bank call loans	203,100
Receivables from brokers	37,588	19,024
Securities failed to deliver	26,694	7,173
Securities borrowed	102,844	99,635
Deposits with clearing organization	82,987	23,297
Other	6,005	267,214
Total payables to brokers, dealers and clearing organizations	238,422	520,708
Payables to customers	413,646	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	124,103	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	148,574	150,000
Fair Value, Estimate Not Practicable, Cash	23,540	79,550
Fair Value, Nonrecurring [Member] | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	19,066
Deposits with clearing organization	82,987	23,297
Fair Value, Estimate Not Practicable, Cash	23,540	79,550
Fair Value, Nonrecurring [Member] | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	2,833	1,316
Receivable from customers	964,772	796,934
Fair Value, Estimate Not Practicable, Investment	60,631	73,971
Clearing organizations	28,463	36,269
Securities loaned	181,745	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,960	43,670
Total Receivable from brokers, dealers and clearing organizations	198,422	163,417
Securities failed to receive	45,593	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	5,079	4,548
Bank call loans	203,100
Receivables from brokers	37,588	19,024
Securities failed to deliver	26,694	7,173
Securities borrowed	102,844	99,635
Other	6,005	267,214
Total payables to brokers, dealers and clearing organizations	238,422	520,708
Payables to customers	413,646	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	124,103	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 140,624	$ 154,988