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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Significant Accounting Policies [Line Items]
Revenues	$ 162,093	$ 164,006	$ 533,891	$ 517,779
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	9,444	4,689	29,553	12,896
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 9,444	4,557	29,544	12,655
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues		$ 132	$ 9	$ 241