Receivables from and payables to brokers, dealers and clearing organizations (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Brokers and Dealers [Abstract]
Receivable from Broker-dealers	$ 409,499	$ 78,898
Securities Failed-to-Deliver	87,820	0
Receivable from Clearing organizations	6,126	0
Stock borrow interest receivable	4,733	0
Receivable from brokers, dealers and clearing organizations	508,178	78,898
Payable to Broker-Dealers	145,860	210,309
Securities Failed-to-Receive	53,540	0
Payable to clearing organizations	48,257	0
Stock loan interest payable	4,496	0
Payable to brokers, dealers and clearing organizations	$ 252,153	$ 210,309