Annual report pursuant to Section 13 and 15(d)

Stock Option and Purchase Plans - Summary of Weighted Average Fair Value of Options Granted (Detail)

v3.8.0.1
Stock Option and Purchase Plans - Summary of Weighted Average Fair Value of Options Granted (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Weighted Average Fair Value of the Options Granted (in usd per share) $ 17.60 $ 16.85 $ 18.00