Receivables from/Payables to Broker-Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets
Due from prime brokers	$ 173,054	$ 101,372
Deposits with clearing organizations	25,862	31,908
Net equity with futures commission merchants	182,496	174,615
Unsettled trades with clearing organization	153,937	102,890
Securities failed to deliver	94,562	65,751
Total receivables from broker-dealers and clearing organizations	629,911	476,536
Liabilities
Due to prime brokers	338,617	294,691
Net equity with futures commission merchants	45,762	46,537
Unsettled trades with clearing organization	51,572	145,376
Securities failed to receive	7
Total payables to broker-dealers and clearing organizations	435,958	486,604
Outstanding principal balance	$ 223,700	$ 219,100