Stockholders' Equity - Summary of the Companys Change in the Common Stock (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Outstanding shares of voting common stock at the beginning of year	37,543,775
Exercise of stock options	380,967	195,410	196,034
Outstanding shares of voting common stock at the end of year	37,620,736	37,543,775
Voting Common Stock [Member]
Class of Stock [Line Items]
Outstanding shares of voting common stock at the beginning of year	37,544,000	37,409,000	37,319,000
Exercise of stock options	262,000	181,000	183,000
Issuance of restricted stock, net of shares withheld for withholding tax payments and cancellations	39,000	104,000	178,000
Repurchases	(224,000)	(150,000)	(271,000)
Outstanding shares of voting common stock at the end of year	37,621,000	37,544,000	37,409,000