Receivables from/Payables to Broker-Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Due from prime brokers	$ 219,573	$ 91,476
Deposits with clearing organizations	112,847	21,995
Net equity with futures commission merchants	203,711	213,030
Unsettled trades with clearing organization	173,778	44,312
Securities failed to deliver	248,088	77,915
Commissions and fees	14,021
Total receivables from broker-dealers and clearing organizations	972,018	448,728
Liabilities
Due to prime brokers	197,439	227,335
Net equity with futures commission merchants	44,526	38,838
Unsettled trades with clearing organization	420,029	429,800
Securities failed to receive	51,143	5
Commissions and fees	3,068
Total payables to broker-dealers and clearing organizations	716,205	695,978
Outstanding principal balance	$ 205,700	$ 309,100