Related Party Transactions - Consolidated Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Revenue:
Revenue	$ 234,606	$ 186,792
Shared Services Fees:
Technology and communications	10,318	10,040
General and administrative	8,340	9,089
Occupancy	3,726	3,639
Affiliated Entity
Operating Income:
Net interest income (expense)		208
Shared Services Fees:
Technology and communications	740	740
General and administrative	24	180
Occupancy	46	155
Transaction fees \| Affiliated Entity
Revenue:
Revenue		59,643
Subscription fees \| Affiliated Entity
Revenue:
Revenue		5,670
Commissions \| Affiliated Entity
Revenue:
Revenue		16,186
Refinitiv market data fees \| Affiliated Entity
Revenue:
Revenue	$ 14,628	$ 13,616

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