Stock-Based Compensation Plans (Summary Of Plans' Restricted Share And Performance-Vested Stock Award Activity) (Detail) - $ / shares	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Restricted Stock
Common Shares
Shares, Beginning of the Period (in shares)	133,425	137,593
Shares, Granted (in shares)	14,249	15,764
Shares, Vested and issued (in shares)	(10,695)	(10,041)
Shares, Forfeited (in shares)	(2,551)	(598)
Shares, Outstanding, End of the Period (in shares)	134,428	142,718
Shares, Vested, but not issuable (in shares)	89,563	88,889
Weighted Average Grant-Date Fair Value
Weighted Average Grant-Date Fair Value (usd per share)	$ 49.94	$ 49.63
Weighted Average Grant-Date Fair Value, Granted (usd per share)	72.53	44.72
Weighted Average Grant-Date Fair Value, Vested and issued (usd per share)	46.03	43.78
Weighted Average Grant-Date Fair Value, Forfeited (usd per share)	52.26	44.26
Weighted Average Grant-Date Fair Value (usd per share)	52.60	49.52
Weighted Average Grant-Date Fair Value, Vested, but not issuable (usd per share)	$ 51.59	$ 51.44
Performance Shares
Common Shares
Shares, Beginning of the Period (in shares)	298,180	276,533
Shares, Granted (in shares)	145,829	118,072
Shares, Vested and issued (in shares)	(68,712)	(78,410)
Shares, Forfeited (in shares)	(14,164)	(13,229)
Shares, Outstanding, End of the Period (in shares)	361,133	302,966
Shares, Vested, but not issuable (in shares)	13,616	6,660
Weighted Average Grant-Date Fair Value
Weighted Average Grant-Date Fair Value (usd per share)	$ 43.64	$ 43.01
Weighted Average Grant-Date Fair Value, Granted (usd per share)	72.60	41.02
Weighted Average Grant-Date Fair Value, Vested and issued (usd per share)	46.85	37.90
Weighted Average Grant-Date Fair Value, Forfeited (usd per share)	52.81	41.12
Weighted Average Grant-Date Fair Value (usd per share)	54.36	43.64
Weighted Average Grant-Date Fair Value, Vested, but not issuable (usd per share)	$ 42.66	$ 37.93

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

No definition available.

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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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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, Weighted Average Grant Date Fair Value

+ References

No definition available.

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