Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		$ 710,668
Fair value of the reverse repurchase agreements		24,006
Securities purchased under agreements to resell	$ 658	24,006
Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	37,664	48,671
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	0
Deposits with clearing organization	7,929	11,748
Securities borrowed	132,368	154,090
Receivables from brokers	19,298	25,768
Securities failed to deliver	9,442	6,172
Clearing organizations	24,361	26,081
Other	930	2,823
Total Receivable from brokers, dealers and clearing organizations	186,399	214,934
Receivable from customers	848,226	847,386
Securities purchased under agreements to resell	658
Fair Value, Estimate Not Practicable, Notes Receivable	40,520	30,099
Drafts payable	42,412	39,228
Bank call loans	118,300	145,800
Securities loaned	180,270	179,875
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	1,567	610
Securities failed to receive	17,559	11,523
Other	10,707	29,381
Total payables to brokers, dealers and clearing organizations	210,103	221,389
Payables to customers	385,907	449,946
Securities sold under agreements to repurchase	586,478	378,084
Senior secured notes	206,380	151,782
Fair Value, Estimate Not Practicable, Investments	65,404	56,300
Level 1
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		456,129
Level 1 | Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	37,664	48,671
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	0
Deposits with clearing organization	7,929	11,748
Drafts payable	42,412	39,228
Level 2
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		169,569
Fair value of the reverse repurchase agreements		24,006
Level 2 | Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Securities borrowed	132,368	154,090
Receivables from brokers	19,298	25,768
Securities failed to deliver	9,442	6,172
Clearing organizations	24,361	26,081
Other	930	2,823
Total Receivable from brokers, dealers and clearing organizations	186,399	214,934
Receivable from customers	848,226	847,386
Securities purchased under agreements to resell	658
Fair Value, Estimate Not Practicable, Notes Receivable	40,520	30,099
Bank call loans	118,300	145,800
Securities loaned	180,270	179,875
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	1,567	610
Securities failed to receive	17,559	11,523
Other	10,707	29,381
Total payables to brokers, dealers and clearing organizations	210,103	221,389
Payables to customers	385,907	449,946
Securities sold under agreements to repurchase	586,478	378,084
Senior secured notes	206,380	151,782
Fair Value, Estimate Not Practicable, Investments	65,404	56,300
Level 3
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		84,970
Carrying Value | Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	37,664	48,671
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	0
Deposits with clearing organization	7,929	11,748
Securities borrowed	132,368	154,090
Receivables from brokers	19,298	25,768
Securities failed to deliver	9,442	6,172
Clearing organizations	24,361	26,081
Other	930	2,823
Total Receivable from brokers, dealers and clearing organizations	186,399	214,934
Receivable from customers	848,226	847,386
Securities purchased under agreements to resell	658
Fair Value, Estimate Not Practicable, Notes Receivable	40,520	30,099
Drafts payable	42,412	39,228
Bank call loans	118,300	145,800
Securities loaned	180,270	179,875
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	1,567	610
Securities failed to receive	17,559	11,523
Other	10,707	29,381
Total payables to brokers, dealers and clearing organizations	210,103	221,389
Payables to customers	385,907	449,946
Securities sold under agreements to repurchase	586,478	378,084
Senior secured notes	200,000	150,000
Fair Value, Estimate Not Practicable, Investments	$ 65,404	$ 56,300