Receivables and Payables (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 263,947		$ 263,947		$ 167,514
Clearing organizations	3,320		3,320		3,480
Securities borrowed	16,214		16,214		866,520
Allowance for doubtful accounts	(469)		(469)		(737)
Total	283,012		283,012		1,036,777
Payables to brokers, dealers and clearing organizations
Broker-dealers	107,421		107,421		64,763
Clearing organizations	12,991		12,991		7,644
Securities loaned	50,867		50,867		888,152
Total	171,279		171,279		960,559
Receivables from customers
Customers	75,324		75,324		49,688
Allowance for doubtful accounts	(455)		(455)		(512)
Total	74,869		74,869		49,176
Payables to customers
Customers	25,606		25,606		9,957
Total	25,606		25,606		9,957
Securities Borrowed and Loaned
Interest earned	3	$ 1,478	1,961	$ 4,337
Interest incurred	(3)	(884)	(1,132)	(3,009)
Interest earned (incurred), net		$ 594	829	$ 1,328
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	16,214		16,214		866,520
Net Amounts Presented in the Consolidated Statement of Financial Condition	16,214		16,214		866,520
Collateral Received or Pledged (including Cash)	16,214		16,214		866,520
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(50,867)		(50,867)		(888,152)
Net Amounts Presented in the Consolidated Statement of Financial Condition	(50,867)		(50,867)		(888,152)
Collateral Received or Pledged (including Cash)	(50,569)		(50,569)		(868,843)
Net Amount	$ (298)		$ (298)		$ (19,309)