Significant Accounting Policies - Summary of Post-Trade Services Revenue by Timing of Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Significant Accounting Policies [Line Items]
Total post-trade services revenues	$ 4,576	$ 2,456
Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Total post-trade services revenues	4,287	2,450
Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Total post-trade services revenues	$ 289	$ 6