Receivables and Payables (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 367,611		$ 367,611		$ 167,514
Clearing organizations	3,982		3,982		3,480
Securities borrowed	12,542		12,542		866,520
Allowance for doubtful accounts	(356)		(356)		(737)
Total	383,779		383,779		1,036,777
Payables to brokers, dealers and clearing organizations
Broker-dealers	153,157		153,157		64,763
Clearing organizations	30,626		30,626		7,644
Securities loaned	75,011		75,011		888,152
Total	258,794		258,794		960,559
Receivables from customers
Customers	175,131		175,131		49,688
Allowance for doubtful accounts	(520)		(520)		(512)
Total	174,611		174,611		49,176
Payables to customers
Customers	73,933		73,933		9,957
Total	73,933		73,933		9,957
Securities Borrowed and Loaned
Interest earned	448	$ 1,451	2,080	$ 2,859
Interest incurred	(185)	(1,080)	(1,251)	(2,126)
Interest earned (incurred), net	263	$ 371	829	$ 733
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	12,542		12,542		866,520
Net Amounts Presented in the Consolidated Statement of Financial Condition	12,542		12,542		866,520
Collateral Received or Pledged (including Cash)	12,542		12,542		866,520
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(75,011)		(75,011)		(888,152)
Net Amounts Presented in the Consolidated Statement of Financial Condition	(75,011)		(75,011)		(888,152)
Collateral Received or Pledged (including Cash)	(74,899)		(74,899)		(868,843)
Net Amount	$ (112)		$ (112)		$ (19,309)