Summary of Significant Accounting Policies - Client Commission Arrangements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Client Commission Arrangements
Client commissions allocated for research	$ 79,300	$ 96,300	$ 100,700
Research receivable, gross	5,400	5,300	3,000
Allowance for research receivable	(100)	(100)	(100)
Research receivable, net of allowance	5,300	5,200	2,900
Accrued research payables	$ 74,175	$ 51,275	$ 45,800