Summary of Information Regarding Stock Options (Detail) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Number of Options
Beginning Balance, number of options | shares	372,561
Exercised, number of options | shares	(134,461)
Forfeited, number of options | shares	(1,200)
Ending Balance, number of options | shares	236,900
Options exercisable at the ending, Number of Options | shares	217,000
Weighted Average Exercise Price per Share
Beginning Balance, Weighted Average Exercise Price per Share | $ / shares	$ 48.24
Exercised, Weighted Average Exercise Price per Share | $ / shares	46.38
Forfeited, Weighted Average Exercise Price per Share | $ / shares	55.67
Ending Balance, Weighted Average Exercise Price per Share | $ / shares	49.25
Options exercisable at September 30, 2017 | $ / shares	$ 48.59
Weighted Average Remaining Contractual Term (years)
Options outstanding at September 30, 2017	4 years
Options exercisable at September 30, 2017	3 years 10 months 24 days
Aggregate Intrinsic Value
Options outstanding at September 30, 2017 | $	$ 11,940
Options exercisable at September 30, 2017 | $	$ 11,081