Receivables from and payables to brokers, dealers and clearing organizations (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Brokers and Dealers [Abstract]
Receivable from Broker-dealers	$ 499,281	$ 409,499
Securities Failed-to-Deliver	30,841	87,820
Receivable from Clearing organizations	26,156	6,126
Stock borrow interest receivable	6,063	4,733
Receivable from brokers, dealers and clearing organizations	562,341	508,178
Payable to Broker-Dealers	188,001	145,860
Securities Failed-to-Receive	40,895	53,540
Payable to clearing organizations	12,700	48,257
Stock loan interest payable	6,025	4,496
Payable to brokers, dealers and clearing organizations	$ 247,621	$ 252,153