Receivables from/Payables to Broker-Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Due from prime brokers	$ 101,372	$ 67,556
Deposits with clearing organizations	31,908	29,595
Net equity with futures commission merchants	174,615	155,060
Unsettled trades with clearing organization	102,890	55,929
Securities failed to deliver	65,751	79,512
Total receivables from broker-dealers and clearing organizations	476,536	387,652
Liabilities
Due to prime brokers	294,691	313,623
Net equity with futures commission merchants	46,537	60,973
Unsettled trades with clearing organization	145,376	311,322
Securities failed to receive		285
Total payables to broker-dealers and clearing organizations	486,604	686,203
Outstanding principal balance	$ 219,100	$ 182,900