Business Segment and Geographic Information - Revenue by Client Sector (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Business Segment and Geographic Information
Total revenue	$ 273,399	$ 234,606
Operating expenses	175,072	156,991
Operating income	98,327	77,615
Institutional
Business Segment and Geographic Information
Total revenue	175,324	145,612
Wholesale
Business Segment and Geographic Information
Total revenue	59,390	48,756
Retail
Business Segment and Geographic Information
Total revenue	18,713	21,676
Market Data
Business Segment and Geographic Information
Total revenue	$ 19,972	$ 18,562