Receivables and Payables (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 184,313		$ 184,313		$ 147,240
Clearing organizations	7,898		7,898		1,447
Securities borrowed	819,051		819,051		496,596
Allowance for doubtful accounts	(927)		(927)		(669)
Total	1,010,335		1,010,335		644,614
Payables to brokers, dealers and clearing organizations
Broker-dealers	94,943		94,943		51,615
Clearing organizations	10,049		10,049		39,433
Securities loaned	849,111		849,111		508,993
Total	954,103		954,103		600,041
Receivables from customers
Customers	106,142		106,142		108,518
Allowance for doubtful accounts	(591)		(591)		(583)
Total	105,551		105,551		107,935
Payables to customers
Customers	31,324		31,324		11,132
Total	31,324		31,324		11,132
Securities Borrowed and Loaned
Fair value of securities borrowed	797,900		797,900
Interest earned	1,451	$ 6,455	2,859	$ 10,858
Interest incurred	(1,080)	(4,337)	(2,126)	(7,403)
Interest earned (incurred), net	371	$ 2,118	733	$ 3,455
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	819,051		819,051		496,596
Net Amounts Presented in the Consolidated Statement of Financial Condition	819,051		819,051		496,596
Collateral Received or Pledged (including Cash)	818,761		818,761		496,374
Net Amount	290		290		222
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(849,111)		(849,111)		(508,993)
Net Amounts Presented in the Consolidated Statement of Financial Condition	(849,111)		(849,111)		(508,993)
Collateral Received or Pledged (including Cash)	(835,260)		(835,260)		(497,462)
Net Amount	$ (13,851)		$ (13,851)		$ (11,531)