Stock-Based Compensation Plans - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Options Outstanding, Beginning balance | shares	1,594,591
Stock Options Outstanding, Granted | shares	200
Stock Options Outstanding, Exercised | shares	(658,400)
Stock Options Outstanding, Expired | shares	(270,979)
Stock Options Outstanding, Ending balance | shares	665,412
Stock Options Outstanding, Exercisable | shares	665,010
Weighted-Average Exercise Price, Outstanding, Beginning balance | $ / shares	$ 139.60
Weighted-Average Exercise Price, Granted | $ / shares	162.57
Weighted-Average Exercise Price, Exercised | $ / shares	116.20
Weighted-Average Exercise Price, Expired | $ / shares	165.46
Weighted-Average Exercise Price, Outstanding, Ending balance | $ / shares	152.23
Weighted-Average Exercise Price, Exercisable | $ / shares	$ 152.23
Weighted-Average Life (In Years), Outstanding, Ending balance	9 months 18 days
Weighted-Average Life (In Years), Exercisable	9 months 18 days
Aggregate Intrinsic Value, Outstanding, Ending balance | $	$ 18,086
Aggregate Intrinsic Value, Exercisable | $	$ 18,072