Significant Accounting Policies - Summary of Post-Trade Services Revenue by Timing of Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Significant Accounting Policies [Line Items]
Revenues	$ 125,490	$ 106,964	$ 249,981	$ 221,678
Post-trade Services [Member]
Significant Accounting Policies [Line Items]
Revenues	3,956	3,620	8,056	8,196
Post-trade Services [Member] | Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Revenues	3,949	$ 3,620	8,036	7,907
Post-trade Services [Member] | Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Revenues	$ 7		$ 20	$ 289