Receivables from and payables to brokers, dealers and clearing organizations (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Brokers and Dealers [Abstract]
Receivable from Broker-dealers	$ 692,581	$ 409,499
Securities Failed-to-Deliver	72,918	87,820
Receivable from Clearing organizations	15,319	6,126
Stock borrow interest receivable	5,295	4,733
Receivable from brokers, dealers and clearing organizations	786,113	508,178
Payable to Broker-Dealers	159,443	145,860
Securities Failed-to-Receive	28,826	53,540
Payable to clearing organizations	36,338	48,257
Stock loan interest payable	4,124	4,496
Payables to brokers, dealers and clearing organizations	$ 228,731	$ 252,153