Stock Option Plan - Summary of Option Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Shares
Outstanding at January 1 (in shares)	1,502,670	1,309,614	1,114,169
Granted (in shares)	250,500	238,000	260,000
Exercised (in shares)	(64,920)	(10,749)	(57,055)
Expired/Forfeited (in shares)	(87,000)	(34,195)	(7,500)
Outstanding December 31 (in shares)	1,601,250	1,502,670	1,309,614
Options exercisable (in shares)	1,098,500	971,545	763,614
Weighted Average Exercise Price
Outstanding at January 1 (in usd per share)	$ 52.86	$ 53.38	$ 52.40
Granted (in usd per share)	43.89	50.00	55.71
Exercised (in usd per share)	45.07	49.19	44.53
Expired/Forfeited (in usd per share)	53.60	54.09	56.07
Outstanding December 31 (in usd per share)	51.73	52.86	53.38
Exercisable at December 31 (in usd per share)	$ 53.22	$ 53.01	$ 52.43
Intrinsic value of options exercised	$ 0.4	$ 0.1	$ 0.6
Closing price (in usd per share)	$ 53.02
Weighted average remaining life of options exercisable	5 years 3 months 18 days
Weighted average remaining life of options outstanding	6 years 2 months 12 days

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.