Receivables from/Payables to Broker-Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets
Due from prime brokers	$ 271,834	$ 91,476
Deposits with clearing organizations	32,513	21,995
Net equity with futures commission merchants	190,415	213,030
Unsettled trades with clearing organization	47,672	44,312
Securities failed to deliver	119,879	77,915
Total receivables from broker-dealers and clearing organizations	662,313	448,728
Liabilities
Due to prime brokers	395,191	227,335
Net equity with futures commission merchants	42,127	38,838
Unsettled trades with clearing organization	152,370	429,800
Securities failed to receive		5
Total payables to broker-dealers and clearing organizations	589,688	695,978
Outstanding principal balance	$ 241,300	$ 309,100