Stock Buy Back Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 02, 2016
Equity Class Of Treasury Stock [Line Items]
Purchase of treasury shares		$ 100,000,000	$ 28,823,000
Class A Common Stock [Member]
Equity Class Of Treasury Stock [Line Items]
Stock repurchase program, authorized amount				$ 100,000,000
Purchase of treasury shares (in shares)	0	2,672,227
Class A Common Stock [Member] | Employee Restricted Stock Plan [Member]
Equity Class Of Treasury Stock [Line Items]
Stock tendered for payments of withholding taxes (in shares)	77,988	73,546
Purchase of treasury shares	$ 3,400,000	$ 2,500,000