Receivables from/Payables to Broker-Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Due from prime brokers	$ 91,476	$ 101,372
Deposits with clearing organizations	21,995	31,908
Net equity with futures commission merchants	213,030	174,615
Unsettled trades with clearing organization	44,312	102,890
Securities failed to deliver	77,915	65,751
Total receivables from broker-dealers and clearing organizations	448,728	476,536
Liabilities
Due to prime brokers	227,335	294,691
Net equity with futures commission merchants	38,838	46,537
Unsettled trades with clearing organization	429,800	145,376
Securities failed to receive	5
Total payables to broker-dealers and clearing organizations	695,978	486,604
Outstanding principal balance	$ 309,100	$ 219,100