Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 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	$ 1,003,267	$ 710,668
Fair value of the reverse repurchase agreements	3,896	24,006
Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	40,590	48,671
Deposits with clearing organization	14,443	11,748
Securities borrowed	160,295	154,090
Receivables from brokers	39,408	25,768
Securities failed to deliver	10,195	6,172
Clearing organizations	20,450	26,081
Other	2,334	2,823
Total Receivable from brokers, dealers and clearing organizations	232,682	214,934
Receivable from customers	830,084	847,386
Drafts payable	32,783	39,228
Bank call loans	195,700	145,800
Securities loaned	142,121	179,875
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	3,153	610
Securities failed to receive	13,173	11,523
Other	16,757	29,381
Total payables to brokers, dealers and clearing organizations	175,204	221,389
Payables to customers	504,348	449,946
Securities sold under agreements to repurchase	456,090	378,084
Senior secured notes	150,282	151,782
Fair Value, Estimate Not Practicable, Investments	59,057	56,300
Level 1 | Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	40,590	48,671
Deposits with clearing organization	14,443	11,748
Drafts payable	32,783	39,228
Level 2
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Fair value of the reverse repurchase agreements	3,896	24,006
Level 2 | Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Securities borrowed	160,295	154,090
Receivables from brokers	39,408	25,768
Securities failed to deliver	10,195	6,172
Clearing organizations	20,450	26,081
Other	2,334	2,823
Total Receivable from brokers, dealers and clearing organizations	232,682	214,934
Receivable from customers	830,084	847,386
Bank call loans	195,700	145,800
Securities loaned	142,121	179,875
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	3,153	610
Securities failed to receive	13,173	11,523
Other	16,757	29,381
Total payables to brokers, dealers and clearing organizations	175,204	221,389
Payables to customers	504,348	449,946
Securities sold under agreements to repurchase	456,090	378,084
Senior secured notes	150,282	151,782
Fair Value, Estimate Not Practicable, Investments	59,057	56,300
Carrying Value | Fair Value, Measurements, Nonrecurring [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	40,590	48,671
Deposits with clearing organization	14,443	11,748
Securities borrowed	160,295	154,090
Receivables from brokers	39,408	25,768
Securities failed to deliver	10,195	6,172
Clearing organizations	20,450	26,081
Other	2,334	2,823
Total Receivable from brokers, dealers and clearing organizations	232,682	214,934
Receivable from customers	830,084	847,386
Drafts payable	32,783	39,228
Bank call loans	195,700	145,800
Securities loaned	142,121	179,875
Fair Value Estimate Not Practicable Payable to Correspondent Brokers	3,153	610
Securities failed to receive	13,173	11,523
Other	16,757	29,381
Total payables to brokers, dealers and clearing organizations	175,204	221,389
Payables to customers	504,348	449,946
Securities sold under agreements to repurchase	456,090	378,084
Senior secured notes	150,000	150,000
Fair Value, Estimate Not Practicable, Investments	$ 59,057	$ 56,300