Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell, at fair value	$ 1,502	$ 658
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	895,251	926,597
Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	5,183	930
Receivable from customers	797,751	848,226
Fair Value, Estimate Not Practicable, Investment	68,585	65,404
Drafts payable	29,242	42,212
Clearing organizations	29,026	24,361
Securities loaned	197,332	180,270
Fair Value, Estimate not Practicable, Due from Employees	43,278	40,520
Total Receivable from brokers, dealers and clearing organizations	203,790	186,399
Securities failed to receive	18,102	17,559
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,004	1,567
Bank call loans	3,000	118,300
Securities purchased under agreements to resell, at fair value	1,502	658
Receivables from brokers	31,396	19,298
Securities failed to deliver	23,390	9,442
Securities borrowed	114,795	132,368
Deposits with clearing organization	22,665	7,929
Other	106	10,707
Total payables to brokers, dealers and clearing organizations	217,544	210,103
Payables to customers	351,750	385,907
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	606,010	586,478
Fair Value, Estimate not Practicable, Senior Secured Notes	202,938	206,380
Fair Value, Estimate Not Practicable, Cash	27,031	37,664
Fair Value, Measurements, Nonrecurring [Member] | Carrying Value [Member]
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	5,183	930
Receivable from customers	797,751	848,226
Fair Value, Estimate Not Practicable, Investment	68,585	65,404
Drafts payable	29,242	42,212
Clearing organizations	29,026	24,361
Securities loaned	197,332	180,270
Fair Value, Estimate not Practicable, Due from Employees	43,278	40,520
Total Receivable from brokers, dealers and clearing organizations	203,790	186,399
Securities failed to receive	18,102	17,559
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,004	1,567
Bank call loans	3,000	118,300
Securities purchased under agreements to resell, at fair value	1,502	658
Receivables from brokers	31,396	19,298
Securities failed to deliver	23,390	9,442
Securities borrowed	114,795	132,368
Deposits with clearing organization	22,665	7,929
Other	106	10,707
Total payables to brokers, dealers and clearing organizations	217,544	210,103
Payables to customers	351,750	385,907
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	606,010	586,478
Fair Value, Estimate not Practicable, Senior Secured Notes	200,000	200,000
Fair Value, Estimate Not Practicable, Cash	27,031	37,664
Fair Value, Measurements, Nonrecurring [Member] | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Drafts payable	29,242	42,212
Deposits with clearing organization	22,665	7,929
Fair Value, Estimate Not Practicable, Cash	27,031	37,664
Fair Value, Measurements, Nonrecurring [Member] | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Other	5,183	930
Receivable from customers	797,751	848,226
Fair Value, Estimate Not Practicable, Investment	68,585	65,404
Clearing organizations	29,026	24,361
Securities loaned	197,332	180,270
Fair Value, Estimate not Practicable, Due from Employees	43,278	40,520
Total Receivable from brokers, dealers and clearing organizations	203,790	186,399
Securities failed to receive	18,102	17,559
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	2,004	1,567
Bank call loans	3,000	118,300
Securities purchased under agreements to resell, at fair value	1,502	658
Receivables from brokers	31,396	19,298
Securities failed to deliver	23,390	9,442
Securities borrowed	114,795	132,368
Other	106	10,707
Total payables to brokers, dealers and clearing organizations	217,544	210,103
Payables to customers	351,750	385,907
Fair Value,Estimate not Practicable, Securities sold under Agreements to Repurchase	606,010	586,478
Fair Value, Estimate not Practicable, Senior Secured Notes	$ 202,938	$ 206,380