Receivables from/Payables to Broker-Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Due from prime brokers	$ 196,487	$ 91,476
Deposits with clearing organizations	156,127	21,995
Net equity with futures commission merchants	187,424	213,030
Unsettled trades with clearing organization	191,212	44,312
Securities failed to deliver	231,135	77,915
Commissions and fees	18,133
Total receivables from broker-dealers and clearing organizations	980,518	448,728
Liabilities
Due to prime brokers	409,323	227,335
Net equity with futures commission merchants	42,634	38,838
Unsettled trades with clearing organization	318,207	429,800
Securities failed to receive	66,194	5
Commissions and fees	2,709
Total payables to broker-dealers and clearing organizations	839,067	695,978
Outstanding principal balance	$ 159,100	$ 309,100