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Stock-Based Compensation - Summary of Activity for RSUs (Detail) - $ / shares	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs - beginning of period	943,379
Granted	259,597	376,081
Vested	(478,238)
Forfeited or expired	(33,889)
RSUs - end of period	689,880
RSUs/SARs, Weighted Average Grant Date Fair Value, Beginning of period	$ 47.57
Weighted Average Grant Date Fair Value, Granted	112.56	$ 67.13
Weighted Average Grant Date Fair Value, Vested	44.21
Weighted Average Grant Date Fair Value, Forfeited or expired	60.77
RSUs/SARs, Weighted Average Grant Date Fair Value, end of period	$ 73.71
Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accrued dividend shares	31
Weighted Average Grant Date Fair Value, Accrued dividend shares	$ 115.65

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- Definition

Share amount for share-based compensation arrangement by share-based payment award equity instruments accrued dividend.

+ References

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Per share amount share based compensation arrangement by share based payment award equity instruments other than options accrued dividend equivalents weighted average grant date fair value.

+ References

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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
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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.

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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).

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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.

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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 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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