Business Segment and Geographic Information - Revenue by Client Sector (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Segment and Geographic Information
Gross revenue	$ 212,870	$ 200,981	$ 659,583	$ 578,258
Total revenue	212,870	200,981	659,583	578,258
Operating expenses	154,349	142,723	467,808	442,768
Operating income	58,521	58,258	191,775	135,490
Institutional
Business Segment and Geographic Information
Gross revenue	131,860	120,858	407,464	341,167
Wholesale
Business Segment and Geographic Information
Gross revenue	44,959	43,040	136,967	124,416
Retail
Business Segment and Geographic Information
Gross revenue	17,453	20,169	59,495	61,314
Market Data
Business Segment and Geographic Information
Gross revenue	$ 18,598	$ 16,914	$ 55,657	$ 51,361