Summary Information for Restricted Stock Awards and Incentive Compensation Stock Awards (Detail) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock - Service Based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Awards, January 1, 2017	307,317
Number of Awards, Awarded	80,181
Number of Awards, Distributed	(68,828)
Number of Awards, Forfeited	(3,830)
Number of Awards, December 31, 2017	314,840	307,317
Weighted-Average Grant-Date Fair Value, January 1, 2017	$ 125.36
Weighted-Average Grant-Date Fair Value, Awarded	213.76	$ 128.48	$ 154.26
Weighted-Average Grant-Date Fair Value, Distributed	121.40
Weighted-Average Grant-Date Fair Value, Forfeited	156.40
Weighted-Average Grant-Date Fair Value, December 31, 2017	$ 151.84	$ 125.36
Restricted Stock - Performance Based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Awards, January 1, 2017	111,317
Number of Awards, Awarded	47,997
Number of Awards, Distributed	(17,692)
Number of Awards, Forfeited	(1,623)
Number of Awards, December 31, 2017	139,999	111,317
Weighted-Average Grant-Date Fair Value, January 1, 2017	$ 122.22
Weighted-Average Grant-Date Fair Value, Awarded	207.73	$ 124.41	108.53
Weighted-Average Grant-Date Fair Value, Distributed	129.14
Weighted-Average Grant-Date Fair Value, Forfeited	150.38
Weighted-Average Grant-Date Fair Value, December 31, 2017	$ 150.34	$ 122.22
Incentive Compensation Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Awards, January 1, 2017	40,549
Number of Awards, Awarded	19,981
Number of Awards, Distributed	(21,784)
Number of Awards, Forfeited	(236)
Number of Awards, December 31, 2017	38,510	40,549
Weighted-Average Grant-Date Fair Value, January 1, 2017	$ 114.44
Weighted-Average Grant-Date Fair Value, Awarded	208.68	$ 124.41	$ 108.53
Weighted-Average Grant-Date Fair Value, Distributed	108.44
Weighted-Average Grant-Date Fair Value, Forfeited	124.41
Weighted-Average Grant-Date Fair Value, December 31, 2017	$ 166.67	$ 114.44

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

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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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