Summary of Stock Option (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options
Beginning balance	2,242,202	2,326,627	2,371,650
Grants	66,850	89,600	394,400
Exercises	(272,325)	(110,375)	(365,223)
Cancellations	(30,750)	(37,750)	(32,200)
Expirations	(230,577)	(25,900)	(42,000)
Ending balance	1,775,400	2,242,202	2,326,627
Options exercisable at End of Period	1,469,967
Weighted-average exercise price
Beginning balance	$ 14.28	$ 14.19	$ 12.75
Grants	15.44	13.99	18.83
Exercises	9.01	8.84	8.63
Cancellations	15.39	15.57	12.23
Expirations	27.16	26.60	26.61
Ending balance	13.44	$ 14.28	$ 14.19
Options exercisable at End of Period	$ 12.85
Weighted-average remaining contractual life (years)
Options outstanding, Ending balance	4 years 8 months 12 days
Options exercisable, Ending balance	4 years 2 months 12 days
Aggregate intrinsic value
Options outstanding, end of period	$ 8,305,300
Options exercisable, end of period	$ 7,667,500