Receivables and Payables - Brokers, Dealers, and Clearing Organizations (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 178,520	$ 189,817
Clearing organizations	40,124	708
Securities borrowed	42,089	4,246
Allowance for doubtful accounts	(751)	(864)
Total	259,982	193,907
Payables to brokers, dealers and clearing organizations
Broker-dealers	143,919	105,022
Clearing organizations	12,357	10,011
Securities loaned	22,610	4,245
Total	$ 178,886	$ 119,278