Summary of PSO Activity (Detail) - Performance Stock Options (PSOs) - 2013 Equity Incentive Plan	9 Months Ended
Sep. 30, 2017 $ / shares shares
Shares Subject to Outstanding Stock Options
Beginning balance	180,752
Options exercised	(4,635)
Ending balance	176,117
Weighted-Average Exercise Price Per Share
Beginning balance \| $ / shares	$ 23.76
Ending balance \| $ / shares	$ 23.76

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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