Stock-Based Compensation - Summary of Nonvested Restricted Stock Awards Activity (Detail)	9 Months Ended
Sep. 30, 2020 $ / shares shares
Time-Based Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Unvested shares of restricted stock outstanding at beginning of period \| shares	742,600
Restricted Stock Awards, Granted \| shares	152,900
Restricted Stock Awards, Vested \| shares	(288,600)
Restricted Stock Awards, Forfeited \| shares	(41,000.0)
Unvested shares of restricted stock outstanding at end of period \| shares	565,900
Unvested shares of restricted stock outstanding, Weighted Average Grant Date Fair Value, at beginning of period	$ 107.93
Granted, Weighted Average Grant Date Fair Value	281.02
Vested, Weighted Average Grant Date Fair Value	98.78
Forfeited, Weighted Average Grant Date Fair Value	171.11
Unvested shares of restricted stock outstanding, Weighted Average Grant Date Fair Value, at end of period	$ 154.78
Market-Based Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Restricted Stock Awards, Granted \| shares	360,900
Restricted Stock Awards, Forfeited \| shares	(3,400)
Unvested shares of restricted stock outstanding at end of period \| shares	357,500
Granted, Weighted Average Grant Date Fair Value	$ 216.69
Vested, Weighted Average Grant Date Fair Value	251.33
Forfeited, Weighted Average Grant Date Fair Value	251.33
Unvested shares of restricted stock outstanding, Weighted Average Grant Date Fair Value, at end of period	$ 216.35

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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.

+ References

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-Publisher FASB
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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

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-Publisher FASB
-Name Accounting Standards Codification
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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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- 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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- Definition

Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

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-Publisher FASB
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- Definition

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

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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.

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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.

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