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Stock-Based Compensation - Summary of Activity for RSUs (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Grant Date Fair Value, Granted	$ 21.74	$ 13.14	$ 8.52
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested RSUs/SARs - beginning of period	943,379
Granted	266,411
Vested	(490,453)
Forfeited or expired	(72,009)
Non-vested RSUs/SARs - end of period	647,394	943,379
RSUs/SARs, Weighted Average Grant Date Fair Value, Beginning of period	$ 47.57
Weighted Average Grant Date Fair Value, Granted	111.64
Weighted Average Grant Date Fair Value, Vested	44.70
Weighted Average Grant Date Fair Value, Forfeited or expired	66.22
RSUs/SARs, Weighted Average Grant Date Fair Value, end of period	$ 74.04	$ 47.57
Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accrued dividend shares	66
Weighted Average Grant Date Fair Value, Accrued dividend shares	$ 94.11

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- Definition

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

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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.

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

+ References

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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 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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