Stock-Based Compensation - Summary Information for Restricted Stock Awards and Incentive Compensation Stock Awards (Detail) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock - Service Based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Awards, January 1, 2018	314,840
Number of Awards, Awarded	64,352
Number of Awards, Distributed	(88,674)
Number of Awards, Forfeited	(12,371)
Number of Awards, December 31, 2018	278,147	314,840
Weighted-Average Grant-Date Fair Value, January 1, 2018	$ 151.84
Weighted-Average Grant-Date Fair Value, Awarded	211.03	$ 213.76	$ 128.48
Weighted-Average Grant-Date Fair Value, Distributed	159.04
Weighted-Average Grant-Date Fair Value, Forfeited	211.63
Weighted-Average Grant-Date Fair Value, December 31, 2018	$ 158.29	$ 151.84
Restricted Stock - Performance Based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Awards, January 1, 2018	139,999
Number of Awards, Awarded	37,138
Number of Awards, Distributed	(20,219)
Number of Awards, Forfeited	(4,979)
Number of Awards, December 31, 2018	151,939	139,999
Weighted-Average Grant-Date Fair Value, January 1, 2018	$ 150.34
Weighted-Average Grant-Date Fair Value, Awarded	212.12	$ 207.73	124.41
Weighted-Average Grant-Date Fair Value, Distributed	126.01
Weighted-Average Grant-Date Fair Value, Forfeited	182.90
Weighted-Average Grant-Date Fair Value, December 31, 2018	$ 174.74	$ 150.34
Incentive Compensation Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Awards, January 1, 2018	38,510
Number of Awards, Awarded	18,180
Number of Awards, Distributed	(20,992)
Number of Awards, Forfeited	(322)
Number of Awards, December 31, 2018	35,376	38,510
Weighted-Average Grant-Date Fair Value, January 1, 2018	$ 166.67
Weighted-Average Grant-Date Fair Value, Awarded	212.12	$ 208.68	$ 124.41
Weighted-Average Grant-Date Fair Value, Distributed	138.54
Weighted-Average Grant-Date Fair Value, Forfeited	184.74
Weighted-Average Grant-Date Fair Value, December 31, 2018	$ 206.55	$ 166.67

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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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

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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).

+ References

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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).

+ References

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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

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-Publisher FASB
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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

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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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-Publisher FASB
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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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-Publisher FASB
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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.

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No definition available.

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