Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 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	$ 502,029	$ 799,719
Fair Value, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	104,870	1,316
Receivable from customers	860,140	796,934
Fair Value, Estimate Not Practicable, Investment	72,103	73,971
Drafts payable	37,374
Clearing organizations	31,361	36,269
Securities loaned	204,339	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,114	43,670
Total Receivable from brokers, dealers and clearing organizations	315,955	163,417
Securities failed to receive	11,528	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	352	4,548
Bank call loans	13,000
Receivables from brokers	28,119	19,024
Securities failed to deliver	24,717	7,173
Securities borrowed	126,888	99,635
Deposits with clearing organization	55,411	23,297
Other	(505)	267,214
Total payables to brokers, dealers and clearing organizations	215,714	520,708
Payables to customers	416,914	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	154,155	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	151,231	154,988
Fair Value, Estimate Not Practicable, Cash	51,237	79,550
Fair Value, Nonrecurring [Member] | Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	104,870	1,316
Receivable from customers	860,140	796,934
Fair Value, Estimate Not Practicable, Investment	72,103	73,971
Drafts payable	37,374
Clearing organizations	31,361	36,269
Securities loaned	204,339	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,114	43,670
Total Receivable from brokers, dealers and clearing organizations	315,955	163,417
Securities failed to receive	11,528	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	352	4,548
Bank call loans	13,000
Receivables from brokers	28,119	19,024
Securities failed to deliver	24,717	7,173
Securities borrowed	126,888	99,635
Deposits with clearing organization	55,411	23,297
Other	(505)	267,214
Total payables to brokers, dealers and clearing organizations	215,714	520,708
Payables to customers	416,914	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	154,155	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	148,574	150,000
Fair Value, Estimate Not Practicable, Cash	51,237	79,550
Fair Value, Nonrecurring [Member] | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	37,374
Deposits with clearing organization	55,411	23,297
Fair Value, Estimate Not Practicable, Cash	51,237	79,550
Fair Value, Nonrecurring [Member] | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	104,870	1,316
Receivable from customers	860,140	796,934
Fair Value, Estimate Not Practicable, Investment	72,103	73,971
Clearing organizations	31,361	36,269
Securities loaned	204,339	234,343
Fair Value, Estimate not Practicable, Due from Employees	44,114	43,670
Total Receivable from brokers, dealers and clearing organizations	315,955	163,417
Securities failed to receive	11,528	14,603
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	352	4,548
Bank call loans	13,000
Receivables from brokers	28,119	19,024
Securities failed to deliver	24,717	7,173
Securities borrowed	126,888	99,635
Other	(505)	267,214
Total payables to brokers, dealers and clearing organizations	215,714	520,708
Payables to customers	416,914	334,735
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	154,155	287,265
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 151,231	$ 154,988