Revenue - Breakdown of Revenues Between Fixed and Variable (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue Recognition
Gross revenue	$ 212,870	$ 200,981	$ 659,583	$ 578,258
Transaction fees
Revenue Recognition
Gross revenue	120,582	112,746	383,552	319,338
Commissions
Revenue Recognition
Gross revenue	39,593	37,590	120,201	108,200
Other
Revenue Recognition
Gross revenue	2,205	2,007	6,658	6,070
Variable
Revenue Recognition
Gross revenue	124,136	115,941	397,671	326,873
Variable | Transaction fees
Revenue Recognition
Gross revenue	94,122	87,574	305,848	246,066
Variable | Subscription Fees including Refinitiv market data fees
Revenue Recognition
Gross revenue	400	440	1,280	1,330
Variable | Commissions
Revenue Recognition
Gross revenue	29,497	27,840	90,079	78,785
Variable | Other
Revenue Recognition
Gross revenue	117	87	464	692
Fixed
Revenue Recognition
Gross revenue	88,734	85,040	261,912	251,385
Fixed | Transaction fees
Revenue Recognition
Gross revenue	26,460	25,172	77,704	73,272
Fixed | Subscription Fees including Refinitiv market data fees
Revenue Recognition
Gross revenue	50,090	48,198	147,892	143,320
Fixed | Commissions
Revenue Recognition
Gross revenue	10,096	9,750	30,122	29,415
Fixed | Other
Revenue Recognition
Gross revenue	$ 2,088	$ 1,920	$ 6,194	$ 5,378