Receivables from and payables to brokers, dealers and clearing organizations (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Brokers and Dealers [Abstract]
Due from Brokers	$ 116,812	$ 78,898
Securities Failed-to-Deliver	26,887	0
Receivables from clearing organizations	1,090	0
Stock borrow interest receivable	596	0
Receivable from brokers, dealers and clearing organizations	145,385	78,898
Payable to Broker-Dealers	219,741	210,309
Securities Failed-to-Receive	45,736	0
Payable to clearing organizations	2,001	0
Stock loan interest payable	456	0
Payable to brokers, dealers and clearing organizations	$ 267,934	$ 210,309