Summary of PSO Activities (Detail) - Performance Stock Options (PSOs) - 2013 Equity Incentive Plan	6 Months Ended
Jun. 30, 2018 $ / shares shares
Shares Subject to Outstanding PSOs
Beginning balance \| shares	164,783
Options exercised \| shares	(9,114)
Ending balance \| shares	155,669
Weighted-Average Exercise Price Per Share
Beginning balance \| $ / shares	$ 23.76
Options exercised \| $ / shares	23.76
Ending balance \| $ / shares	$ 23.76

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

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

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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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