Stock-Based Compensation Plans - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Options Outstanding, Beginning balance | shares	665,412
Stock Options Outstanding, Granted | shares	116,852
Stock Options Outstanding, Exercised | shares	(535,724)
Stock Options Outstanding, Expired | shares	(25,949)
Stock Options Outstanding, Ending balance | shares	220,591
Stock Options Outstanding, Exercisable | shares	103,725
Weighted-Average Exercise Price, Outstanding, Beginning balance | $ / shares	$ 152.23
Weighted-Average Exercise Price, Granted | $ / shares	190.78
Weighted-Average Exercise Price, Exercised | $ / shares	154.91
Weighted-Average Exercise Price, Expired | $ / shares	221.81
Weighted-Average Exercise Price, Outstanding, Ending balance | $ / shares	157.98
Weighted-Average Exercise Price, Exercisable | $ / shares	$ 121.11
Weighted-Average Life (In Years), Outstanding, Ending balance	5 years 3 months 18 days
Weighted-Average Life (In Years), Exercisable	5 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding, Ending balance | $	$ 2,739
Aggregate Intrinsic Value, Exercisable | $	$ 2,737