Stock Option Plan - Summary of Option Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares
Outstanding at January 1 (in shares)	1,114,169	913,320	833,630
Granted (in shares)	260,000	245,000	232,500
Exercised (in shares)	(57,055)	(39,151)	(149,060)
Expired/Forfeited (in shares)	(7,500)	(5,000)	(3,750)
Outstanding December 31 (in shares)	1,309,614	1,114,169	913,320
Options exercisable (in shares)	763,614	600,919	430,945
Weighted Average Exercise Price
Outstanding at January 1 (in usd per share)	$ 52.40	$ 52.80	$ 49.92
Granted (in usd per share)	55.71	49.46	59.41
Exercised (in usd per share)	44.53	42.98	46.97
Expired/Forfeited (in usd per share)	56.07	54.78	53.73
Outstanding December 31 (in usd per share)	53.38	52.40	52.80
Exercisable at December 31 (in usd per share)	$ 52.43	$ 50.93	$ 48.94
Intrinsic value of options exercised	$ 0.6	$ 0.5	$ 2.2
Intrinsic value of options outstanding	2.5
Intrinsic value of options exercisable	$ 2.0
Closing price (in usd per share)	$ 52.78
Weighted average remaining life of options exercisable	5 years 10 months 24 days
Weighted average remaining life of options outstanding	7 years

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.