Receivables from and payables to brokers, dealers and clearing organizations (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Brokers and Dealers [Abstract]
Receivable from Broker-dealers	$ 199,747	$ 78,898
Securities Failed-to-Deliver	33,877	0
Receivable from Clearing organizations	9,671	0
Stock borrow interest receivable	3,841	0
Receivable from brokers, dealers and clearing organizations	247,136	78,898
Payable to Broker-Dealers	118,568	210,309
Securities Failed-to-Receive	26,315	0
Payable to clearing organizations	9,000	0
Stock loan interest payable	2,220	0
Payable to brokers, dealers and clearing organizations	$ 156,103	$ 210,309