Stockholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jul. 13, 2017
Schedule Of Capitalization Equity [Line Items]
Common stock, voting rights	Class B common stock is entitled to ten votes per share as compared to one vote per share for the Class A common stock
Cash dividend paid per share	$ 0.16	$ 0.13
Aggregate amount of cash dividends paid	$ 14,700,000	$ 11,200,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Amount approved under share purchase			$ 15,000,000
Aggregate purchase price of repurchased shares	$ 5,330,000
Class A common stock
Schedule Of Capitalization Equity [Line Items]
Number of shares repurchased	1,000,000
Average price of repurchased shares	$ 5.54
Aggregate purchase price of repurchased shares	$ 5,300,000