Stockholders' Equity (Share Repurchase Agreement) - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 03, 2017	Mar. 03, 2017
Class of Stock [Line Items]
Repurchase and retirement of common stock, value	$ 21,318	$ 464	$ 792
Share Repurchase Plan [Member]
Class of Stock [Line Items]
Repurchase and retirement of common stock, value	15,154	$ 20,026	$ 1,610
Institutional Investors [Member] | Share Repurchase Plan [Member]
Class of Stock [Line Items]
Repurchase and retirement of common stock, value	$ 20,345
Repurchase and retirement of common stock, shares	413,600
Common stock repurchased and retired, price per share	$ 49.19
4.25% Convertible Senior Notes [Member]
Class of Stock [Line Items]
Debt instrument stated interest rate	4.25%			4.25%	4.25%