Summary of Information Regarding Stock Options (Detail) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Number of Options
Beginning Balance, number of options | shares	189,040
Exercised, number of options | shares	(40,418)
Ending Balance, number of options | shares	148,622
Options exercisable at the ending, Number of Options | shares	147,622
Weighted Average Exercise Price per Share
Beginning Balance, Weighted Average Exercise Price per Share | $ / shares	$ 49.34
Exercised, Weighted Average Exercise Price per Share | $ / shares	48.42
Ending Balance, Weighted Average Exercise Price per Share | $ / shares	49.59
Options exercisable at March 31, 2018 | $ / shares	$ 49.53
Weighted Average Remaining Contractual Term (years)
Options outstanding at March 31, 2018	3 years 8 months 12 days
Options exercisable at March 31, 2018	3 years 8 months 12 days
Aggregate Intrinsic Value
Options outstanding at March 31, 2018 | $	$ 8,927
Options exercisable at March 31, 2018 | $	$ 8,875