Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Stock Option Activity and Related Information (Detail)

v3.8.0.1
Stock Based Compensation - Stock Option Activity and Related Information (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Outstanding at beginning of year, Options 95,706 95,706 115,606
Granted, Options     11,000
Exercised, Options (1,100) 0 (30,900)
Outstanding at end of year, Options 94,606 95,706 95,706
Exercisable at end of year, Options 93,106 92,706 63,815
Outstanding at beginning of year, Weighted average exercise price $ 52.08 $ 52.08 $ 48.54
Granted, Weighted average exercise price     91.58
Exercised, Weighted average exercise price 39.00   52.87
Outstanding at end of year, Weighted average exercise price 52.24 52.08 52.08
Exercisable at end of year, Weighted average exercise price $ 51.85 $ 51.31 $ 48.73