Stockholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended		18 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Apr. 11, 2018	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	$ 0.20	$ 0.16
Aggregate amount of cash dividends paid	$ 17,800,000	$ 14,700,000
Preferred stock, shares issued	0	0	0
Preferred stock, shares outstanding	0	0	0
Amount approved under share purchase				$ 30,000,000	$ 15,000,000
Aggregate purchase price of repurchased shares	$ 13,812,000	$ 5,330,000
Class A common stock
Schedule Of Capitalization Equity [Line Items]
Amount approved under share purchase				$ 15,000,000
Number of shares repurchased	3,500,000		4,500,000
Average price of repurchased shares	$ 3.88		$ 4.23
Aggregate purchase price of repurchased shares	$ 13,700,000		$ 19,100,000