Summary Information for Stock Options (Detail)	12 Months Ended
Dec. 31, 2017 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding at January 1, 2017 \| shares	406,891
Number of Options, Exercised \| shares	(104,093)
Number of Options, Terminated \| shares	(840)
Number of Options, Outstanding at December 31, 2017 \| shares	301,958
Number of Options, Exercisable at December 31, 2017 \| shares	261,380
Weighted-Average Exercise Price, Outstanding at January 1, 2017 \| $ / shares	$ 93.27
Weighted-Average Exercise Price, Exercised \| $ / shares	98.68
Weighted-Average Exercise Price, Terminated \| $ / shares	80.52
Weighted-Average Exercise Price, Outstanding at December 31, 2017 \| $ / shares	91.44
Weighted-Average Exercise Price, Exercisable at December 31, 2017 \| $ / shares	$ 83.37
Weighted-Average Remaining Contractual Life (years), Outstanding at December 31, 2017	3 years 7 months 6 days
Weighted-Average Remaining Contractual Life (years), Exercisable at December 31, 2017	3 years 1 month 6 days

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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- Definition

For presentations that combine terminations, 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 or that expired.

+ References

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Weighted average price of options that were either forfeited or expired.

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- Definition

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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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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