Share-Based Compensation - Summary of RSUs Outstanding Under LTIP (Detail)	3 Months Ended
Mar. 31, 2019 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested as of January 1, 2019	230,922
Granted	71,324
Vested	(77,213)
Forfeited	(7,821)
Nonvested as of March 31, 2019	217,212
Nonvested as of January 1, 2018, Weighted-Average Grant-Date Fair Value \| $ / shares	$ 76.20
Weighted-Average Grant-Date Fair Value, Granted \| $ / shares	56.44
Weighted-Average Grant-Date Fair Value, Vested \| $ / shares	34.16
Weighted-Average Grant-Date Fair Value, Forfeited \| $ / shares	68.95
Nonvested as of March 31, 2018, Weighted-Average Grant-Date Fair Value \| $ / shares	$ 60.98
Performance and Service Required [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested as of January 1, 2019	213,624
Granted	63,201
Vested	(75,921)
Forfeited	(6,753)
Nonvested as of March 31, 2019	194,151
Service Required [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested as of January 1, 2019	17,298
Granted	8,123
Vested	(1,292)
Forfeited	(1,068)
Nonvested as of March 31, 2019	23,061

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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-Publisher FASB
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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).

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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.

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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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- 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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No definition available.

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