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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Significant Accounting Policies [Line Items]
Revenues	$ 131,597	$ 101,443	$ 381,578	$ 323,121
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	3,784	3,475	11,840	11,671
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	3,775	3,463	11,811	11,370
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 9	$ 12	$ 29	$ 301