Stock Option Plan - Summary of Option Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Outstanding at January 1 (in shares)	833,630	860,274	748,208
Granted (in shares)	232,500	226,500	225,000
Exercised (in shares)	(149,060)	(246,894)	(112,934)
Expired/Forfeited (in shares)	(3,750)	(6,250)	0
Outstanding December 31 (in shares)	913,320	833,630	860,274
Options exercisable (in shares)	430,945	375,255	435,899
Weighted Average Exercise Price
Outstanding at January 1 (in usd per share)	$ 49.92	$ 46.58	$ 44.79
Granted (in usd per share)	59.41	57.74	51.07
Exercised (in usd per share)	46.97	45.59	43.67
Expired/Forfeited (in usd per share)	53.73	45.31	0.00
Outstanding December 31 (in usd per share)	52.80	49.92	46.58
Exercisable at December 31 (in usd per share)	$ 48.94	$ 46.68	$ 45.33
Intrinsic value of options exercised	$ 2.2	$ 3.4	$ 1.5
Intrinsic value of options outstanding	8.2
Intrinsic value of options exercisable	$ 5.5
Closing price (in usd per share)	$ 61.75
Weighted average remaining life of options exercisable	6 years 5 months 13 days
Weighted average remaining life of options outstanding	7 years 5 months 24 days