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Stock-Based Compensation Plans - Summary of Transactions of the Company's Stock Option Plans (Detail) - Stock Options Plan [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Number of Options, Beginning balance | shares	570,994
Exercised, Number of Options | shares	(295,809)
Forfeited, Number of Options | shares	(16,026)
Outstanding, Number of Options, Ending balance | shares	357,666
Outstanding, Weighted Average Exercise Price, Beginning balance	$ 20.38
Exercised, Weighted Average Exercise Price	27.04
Forfeited, Weighted Average Exercise Price	13.18
Outstanding, Weighted Average Exercise Price, Ending balance	13.74
Outstanding, Weighted Average Grant-Date Fair Value	21.68
Exercised	14.98
Forfeited, Weighted Average Grant-Date Fair Value	25.93
Outstanding, Weighted Average Grant-Date Fair Value	$ 35.69
Discount [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Number of Options | shares	79,664
Granted, Weighted Average Exercise Price	$ 0.00
Granted, Weighted Average Grant-Date Fair Value	$ 61.39
Market Value [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Number of Options | shares	18,843
Granted, Weighted Average Exercise Price	$ 70.60
Granted, Weighted Average Grant-Date Fair Value	$ 16.84

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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