Stock-Based Compensation Plans (Summary Of Plans' Restricted Share And Performance-Vested Stock Award Activity) (Detail) - $ / shares	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Restricted shares
Common Shares
Outstanding Beginning of the Period (in shares)	144,328	143,263
Granted (in shares)	113,156	13,099
Vested and issued (in shares)	(13,917)	(15,257)
Expired, canceled or forfeited (in shares)	(6,547)	(1,691)
Outstanding End of the Period (in shares)	237,020	139,414
Vested, but deferred (in shares)	93,514	91,505
Weighted Average Grant-Date Fair Value
Beginning Weighted Average Grant-Date Fair Value (usd per share)	$ 60.37	$ 60.80
Weighted Average Grant-Date Fair Value, Granted (usd per share)	61.32	71.66
Weighted Average Grant-Date Fair Value, Vested and issued (usd per share)	77.77	76.69
Weighted Average Grant-Date Fair Value, Expired, canceled or forfeited (usd per share)	73.71	79.50
Ending Weighted Average Grant-Date Fair Value (usd per share)	59.43	59.85
Weighted Average Grant-Date Fair Value, Vested, but deferred (usd per share)	$ 52.08	$ 52.13
Performance-based stock
Common Shares
Outstanding Beginning of the Period (in shares)	465,515	396,855
Granted (in shares)	169,864	175,696
Added by performance factor at vesting (in shares)	48,831	33,950
Vested and issued (in shares)	(180,789)	(128,238)
Expired, canceled or forfeited (in shares)	(18,727)	(10,430)
Outstanding End of the Period (in shares)	484,694	467,833
Vested, but deferred (in shares)	34,441	33,701
Weighted Average Grant-Date Fair Value
Beginning Weighted Average Grant-Date Fair Value (usd per share)	$ 74.37	$ 67.71
Weighted Average Grant-Date Fair Value, Granted (usd per share)	63.62	71.57
Weighted Average Grant-Date Fair Value, Added by performance factor at vesting (usd per share)	72.59	40.99
Weighted Average Grant-Date Fair Value, Vested and issued (usd per share)	72.59	41.00
Weighted Average Grant-Date Fair Value, Expired, canceled or forfeited (usd per share)	72.66	74.70
Ending Weighted Average Grant-Date Fair Value (usd per share)	71.15	74.38
Weighted Average Grant-Date Fair Value, Vested, but deferred (usd per share)	$ 43.07	$ 42.92

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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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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- Definition

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.

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

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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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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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