Related Party Transactions - Consolidated Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue:
Revenue	$ 212,870	$ 200,981	$ 659,583	$ 578,258
Operating Income and Expenses:
Interest income	(349)	636	64	1,669
Shared Services Fees:
Technology and communications	12,037	9,527	34,397	29,086
General and administrative	8,657	7,507	24,520	25,961
Occupancy	3,443	3,640	10,678	10,900
Affiliated Entity
Operating Income and Expenses:
Interest income	0	0	0	858
Shared Services Fees:
Technology and communications	740	740	2,220	2,220
General and administrative	0	24	(596)	406
Occupancy	0	123	15	434
Transaction fees | Affiliated Entity
Revenue:
Revenue	0	0	0	59,643
Subscription fees | Affiliated Entity
Revenue:
Revenue	0	0	0	5,670
Commissions | Affiliated Entity
Revenue:
Revenue	0	0	0	16,186
Refinitiv market data fees | Affiliated Entity
Revenue:
Revenue	$ 14,273	$ 13,251	$ 43,466	$ 40,252