Significant Accounting Policies - Summary of Information 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 information services revenues	$ 7,066	$ 6,185
Transferred over Time [Member]
Significant Accounting Policies [Line Items]
Total information services revenues	6,842	5,957
Transferred at a Point in Time [Member]
Significant Accounting Policies [Line Items]
Total information services revenues	$ 224	$ 228