Significant Accounting Policies - Summary of Post-Trade Services Revenue by Timing of Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Significant Accounting Policies [Line Items]
Revenues	$ 164,006	$ 131,597	$ 517,779	$ 381,578
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	4,689	3,784	12,896	11,840
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	4,557	3,775	12,655	11,811
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 132	$ 9	$ 241	$ 29