Related Party Transactions - Consolidated Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Financial Designation, Predecessor and Successor [Fixed List]		Predecessor		Predecessor
Revenues
Revenue	$ 200,981	$ 167,790	$ 578,258	$ 478,941
Expenses
Technology and communications	9,527	9,112	29,086	26,598
General and administrative	7,507	9,386	25,961	23,056
Occupancy	3,640	3,491	10,900	$ 10,732
Affiliated Entity
Related Party Transaction [Line Items]
Financial Designation, Predecessor and Successor [Fixed List]				Predecessor
Operating Income
Interest income		6	858	$ 34
Expenses
Technology and communications	740	740	2,220	2,220
General and administrative	24	180	406	539
Occupancy	123	155	434	466
Transaction fees \| Affiliated Entity
Revenues
Revenue		52,796	59,643	159,663
Subscription fees \| Affiliated Entity
Revenues
Revenue		5,446	5,670	16,627
Commissions \| Affiliated Entity
Revenues
Revenue		11,808	16,186	34,944
Refinitiv market data fees \| Affiliated Entity
Revenues
Revenue	$ 13,251	$ 12,533	$ 40,252	$ 36,851

The amount of expense in the period for communications and data processing expense.

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