Receivables from and payables to brokers, dealers and clearing organizations (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Brokers and Dealers [Abstract]
Receivable from Broker-dealers	$ 1,263,635	$ 623,523
Securities Failed-to-Deliver	71,280	45,673
Clearing organizations	49,978	3,180
Securities borrowed interest receivable	5,928	9,319
Receivable from brokers, dealers and clearing organizations	1,390,821	681,695
Payable to Broker-Dealers	228,628	185,838
Securities Failed-to-Receive	39,496	57,580
Clearing organizations	39,077	18,063
Securities loaned interest payable	5,649	9,537
Payables to brokers, dealers and clearing organizations	$ 312,850	$ 271,018