Stock-Based Compensation Plans (Summary Of Plans' Restricted Share And Performance-Vested Stock Award Activity) (Detail) - $ / shares	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Restricted Stock
Common Shares
Outstanding Beginning of the Period (in shares)	127,787	133,425
Granted (in shares)	28,506	14,249
Vested and issued (in shares)	(10,438)	(10,695)
Forfeited or canceled (in shares)	(982)	(2,551)
Outstanding End of the Period (in shares)	144,873	134,428
Vested, but not issuable (in shares)	90,294	89,563
Weighted Average Grant-Date Fair Value
Beginning Weighted Average Grant-Date Fair Value (usd per share)	$ 53.33	$ 49.94
Weighted Average Grant-Date Fair Value, Granted (usd per share)	87.65	72.53
Weighted Average Grant-Date Fair Value, Vested and issued (usd per share)	56.88	46.03
Weighted Average Grant-Date Fair Value, Forfeited or canceled (usd per share)	55.39	52.26
Ending Weighted Average Grant-Date Fair Value (usd per share)	59.82	52.60
Weighted Average Grant-Date Fair Value, Vested, but not issuable (usd per share)	$ 51.88	$ 51.59
Performance Shares
Common Shares
Outstanding Beginning of the Period (in shares)	359,196	298,180
Granted (in shares)	134,326	145,829
Vested and issued (in shares)	(82,307)	(68,712)
Forfeited or canceled (in shares)	(13,582)	(14,164)
Outstanding End of the Period (in shares)	397,633	361,133
Vested, but not issuable (in shares)	21,477	13,616
Weighted Average Grant-Date Fair Value
Beginning Weighted Average Grant-Date Fair Value (usd per share)	$ 54.37	$ 43.64
Weighted Average Grant-Date Fair Value, Granted (usd per share)	88.28	72.60
Weighted Average Grant-Date Fair Value, Vested and issued (usd per share)	44.39	46.85
Weighted Average Grant-Date Fair Value, Forfeited or canceled (usd per share)	59.18	52.81
Ending Weighted Average Grant-Date Fair Value (usd per share)	67.72	54.36
Weighted Average Grant-Date Fair Value, Vested, but not issuable (usd per share)	$ 43.52	$ 42.66

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

+ References

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

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+ References

No definition available.

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- Definition

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

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