Significant Accounting Policies - Summary of Post-Trade Services Revenue by Timing of Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Significant Accounting Policies [Line Items]
Revenues	$ 195,464	$ 168,978
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	10,261	4,153
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	10,020	4,150
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 241	$ 3