Revenue - Breakdown of Revenues Between Fixed and Variable (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue Recognition
Gross revenues	$ 234,606	$ 186,792
Transaction fees
Revenue Recognition
Gross revenues	140,824	102,640
Commissions
Revenue Recognition
Gross revenues	42,493	34,197
Other
Revenue Recognition
Gross revenues	2,178	1,894
Variable
Revenue Recognition
Gross revenues	148,353	103,983
Variable | Transaction fees
Revenue Recognition
Gross revenues	115,337	78,915
Variable | Subscription Fees including Refinitiv market data
Revenue Recognition
Gross revenues	465	455
Variable | Commissions
Revenue Recognition
Gross revenues	32,441	24,310
Variable | Other
Revenue Recognition
Gross revenues	110	303
Fixed
Revenue Recognition
Gross revenues	86,253	82,809
Fixed | Transaction fees
Revenue Recognition
Gross revenues	25,487	23,725
Fixed | Subscription Fees including Refinitiv market data
Revenue Recognition
Gross revenues	48,646	47,606
Fixed | Commissions
Revenue Recognition
Gross revenues	10,052	9,887
Fixed | Other
Revenue Recognition
Gross revenues	$ 2,068	$ 1,591