Significant Accounting Policies - Summary of Post-Trade Services Revenue by Timing of Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Significant Accounting Policies [Line Items]
Revenues	$ 168,978	$ 124,491
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	4,153	4,100
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	4,150	4,087
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 3	$ 13