Stock-Based Compensation Plans (Summary of Plans' Restricted and Performance Share Award Activity) (Detail) - $ / shares	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022
Restricted shares
Common Shares
Outstanding, beginning of the period (in shares)	610,155	476,813
Granted (in shares)	260,173	204,043
Vested and issued (in shares)	(103,161)	(56,380)
Expired, canceled or forfeited (in shares)	(7,638)	(9,722)
Outstanding, end of the period (in shares)	759,529	614,754
Vested, but deferred (in shares)	98,247	96,400
Weighted Average Grant-Date Fair Value
Outstanding, beginning of period (usd per share)	$ 73.21	$ 61.33
Granted (usd per share)	88.29	96.95
Vested and issued (usd per share)	64.08	64.44
Canceled or forfeited (usd per share)	82.82	72.87
Outstanding, end of period (usd per share)	79.52	72.68
Vested, but deferred (usd per share)	$ 53.35	$ 52.83
Performance-based stock
Common Shares
Outstanding, beginning of the period (in shares)	545,379	557,255
Granted (in shares)	186,313	160,326
Added by performance factor at vesting (in shares)	23,161	0
Vested and issued (in shares)	(178,203)	0
Expired, canceled or forfeited (in shares)	(7,909)	(163,803)
Outstanding, end of the period (in shares)	568,741	553,778
Vested, but deferred (in shares)	36,438	35,424
Weighted Average Grant-Date Fair Value
Outstanding, beginning of period (usd per share)	$ 70.30	$ 62.94
Granted (usd per share)	92.45	97.16
Added by performance factor at vesting (usd per share)	63.64	0
Vested and issued (usd per share)	63.64	0
Canceled or forfeited (usd per share)	82.73	71.44
Outstanding, end of period (usd per share)	79.20	70.34
Vested, but deferred (usd per share)	$ 44.99	$ 44.04

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(ii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

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- Definition

Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Added By Performance Factor At Vesting

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested but not Issuable in Period

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested but not Issuable in Period, Weighted Average Grant Date Fair Value

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No definition available.

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