Receivables from and payables to brokers, dealers and clearing organizations (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Brokers and Dealers [Abstract]
Receivable from Broker-dealers	$ 569,998	$ 409,499
Securities Failed-to-Deliver	47,014	87,820
Receivable from Clearing organizations	16,541	6,126
Stock borrow interest receivable	10,839	4,733
Receivable from brokers, dealers and clearing organizations	644,392	508,178
Payable to Broker-Dealers	131,358	145,860
Securities Failed-to-Receive	54,539	53,540
Payable to clearing organizations	18,202	48,257
Stock loan interest payable	6,227	4,496
Payables to brokers, dealers and clearing organizations	$ 210,326	$ 252,153