Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) shares in Thousands	9 Months Ended
Sep. 30, 2018 $ / shares shares
Shares
Outstanding at beginning of period \| shares	3,755
Granted \| shares	1,416
Exercised \| shares	(535)
Canceled or forfeited \| shares	(97)
Outstanding at end of period \| shares	4,539
Exercisable at end of period \| shares	2,035
Vested and expected to vest at end of period \| shares	4,507
Weighted-average exercise price per share
Outstanding at beginning of period \| $ / shares	$ 67.91
Granted \| $ / shares	193.00
Exercised \| $ / shares	52.87
Canceled or forfeited \| $ / shares	134.63
Outstanding at end of period \| $ / shares	107.28
Exercisable at end of period \| $ / shares	60.91
Vested and expected to vest at end of period \| $ / shares	$ 107.32

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.

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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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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.

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

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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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-Publisher FASB
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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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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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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 per share amount at which grantees can acquire shares of common stock by exercise of options.

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

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