Summary of Significant Accounting Policies - Client Commission Arrangements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Client Commission Arrangements
Client commissions allocated for research	$ 96,300	$ 100,700	$ 110,900
Research receivable, gross	5,300	3,000	3,500
Allowance for research receivable	(100)	(100)	(100)
Research receivable, net of allowance	5,200	2,900	3,400
Accrued research payables	$ 51,275	$ 45,826	$ 46,300