Share Repurchase Program - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018	Feb. 06, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Stock repurchase program expiration date	Feb. 06, 2021
Class A Common Stock [Member]
Class of Stock [Line Items]
Common Stock, par value	$ 0.001		$ 0.001
Stock repurchase program authorized amount		$ 150.0
Class A Common Stock [Member] | Share Repurchase Program [Member]
Class of Stock [Line Items]
Common Stock, par value		$ 0.001
Stock repurchase program remaining authorized repurchase amount	$ 92.0