Summary of Significant Accounting Policies - Client Commission Arrangements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Client Commission Arrangements
Client commissions allocated for research	$ 100,700	$ 110,900	$ 119,400
Prepaid research, gross	3,000	3,500	4,300
Allowance for prepaid research	(100)	(100)
Prepaid research, net of allowance	2,900	3,400	4,300
Accrued research payables	$ 45,826	$ 46,274	$ 56,700