Significant Accounting Policies - Summary of Post-Trade Services Revenue by Timing of Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Significant Accounting Policies [Line Items]
Revenues	$ 184,795	$ 125,490	$ 353,773	$ 249,981
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	4,054	3,956	8,207	8,056
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	3,948	3,949	8,098	8,036
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 106	$ 7	$ 109	$ 20