Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Prior period reclassification adjustment	$ 1.6
Sales and marketing [Member]
Prior period reclassification adjustment		$ 0.7
General and administrative [Member]
Prior period reclassification adjustment		$ 0.1