Receivables from/Payables to Broker-Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets
Due from prime brokers	$ 81,468	$ 91,476
Deposits with clearing organizations	23,751	21,995
Net equity with futures commission merchants	167,922	213,030
Unsettled trades with clearing organization	67,694	44,312
Securities failed to deliver	44,533	77,915
Total receivables from broker-dealers and clearing organizations	385,368	448,728
Liabilities
Due to prime brokers	149,555	227,335
Net equity with futures commission merchants	42,440	38,838
Unsettled trades with clearing organization	205,828	429,800
Securities failed to receive	81	5
Total payables to broker-dealers and clearing organizations	397,904	695,978
Outstanding principal balance	$ 160,300	$ 309,100