Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 646,680	$ 799,719
Fair Value, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,184	1,316
Receivable from customers	1,095,035	796,934
Fair Value, Estimate Not Practicable, Investment	75,785	73,971
Drafts payable	18,251
Clearing organizations	34,165	36,269
Securities loaned	291,969	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,723	43,670
Total Receivable from brokers, dealers and clearing organizations	207,201	163,417
Securities failed to receive	19,421	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,280	4,548
Bank call loans	156,900
Receivables from brokers	29,088	19,024
Securities failed to deliver	14,565	7,173
Securities borrowed	128,199	99,635
Deposits with clearing organization	41,858	23,297
Other	53,163	267,214
Total payables to brokers, dealers and clearing organizations	366,833	520,708
Payables to customers	380,746	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	252,827	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	126,111	154,988
Fair Value, Estimate Not Practicable, Cash	32,080	79,550
Fair Value, Nonrecurring [Member] | Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,184	1,316
Receivable from customers	1,095,035	796,934
Fair Value, Estimate Not Practicable, Investment	75,785	73,971
Drafts payable	18,251
Clearing organizations	34,165	36,269
Securities loaned	291,969	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,723	43,670
Total Receivable from brokers, dealers and clearing organizations	207,201	163,417
Securities failed to receive	19,421	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,280	4,548
Bank call loans	156,900
Receivables from brokers	29,088	19,024
Securities failed to deliver	14,565	7,173
Securities borrowed	128,199	99,635
Deposits with clearing organization	41,858	23,297
Other	53,163	267,214
Total payables to brokers, dealers and clearing organizations	366,833	520,708
Payables to customers	380,746	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	252,827	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	125,000	150,000
Fair Value, Estimate Not Practicable, Cash	32,080	79,550
Fair Value, Nonrecurring [Member] | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	18,251
Deposits with clearing organization	41,858	23,297
Fair Value, Estimate Not Practicable, Cash	32,080	79,550
Fair Value, Nonrecurring [Member] | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	1,184	1,316
Receivable from customers	1,095,035	796,934
Fair Value, Estimate Not Practicable, Investment	75,785	73,971
Clearing organizations	34,165	36,269
Securities loaned	291,969	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,723	43,670
Total Receivable from brokers, dealers and clearing organizations	207,201	163,417
Securities failed to receive	19,421	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,280	4,548
Bank call loans	156,900
Receivables from brokers	29,088	19,024
Securities failed to deliver	14,565	7,173
Securities borrowed	128,199	99,635
Other	53,163	267,214
Total payables to brokers, dealers and clearing organizations	366,833	520,708
Payables to customers	380,746	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	252,827	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 126,111	$ 154,988