Equity-based compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Stock Options
Stock Options, Outstanding at beginning of period (in shares)	918,206
Stock Options, Granted (in shares)	224,141
Stock Options, Exercised (in shares)	(67,349)
Stock Options, Forfeited (in shares)	(60,793)
Stock Options, Outstanding at end of period (in shares)	1,014,205
Stock Options, Vested or expected to vest (in shares)	1,014,205
Stock Option, Exercisable (in shares)	256,970
Weighted average exercise price
Weighted average exercise price, Outstanding at beginning of period (usd per share)	$ 19.59
Weighted average exercise price, Granted (usd per share)	38.83
Weighted average exercise price, Exercised (usd per share)	18.04
Weighted average exercise price, Forfeited (usd per share)	25.02
Weighted average exercise price, Outstanding at end of period (usd per share)	23.62
Weighted average exercise price, Vested or expected to vest (usd per share)	23.62
Weighted average exercise price, Exercisable (usd per share)	$ 19.04
Weighted average remaining contractual term, Outstanding at end of period (years)	8 years
Weighted average remaining contractual term, Vested or expected to vest (years)	8 years
Weighted average remaining contractual term, Exercisable (years)	7 years 4 months
Aggregate intrinsic value, Outstanding		$ 30,427
Aggregate intrinsic value, Vested or expected to vest		30,427
Aggregate intrinsic value, Exercisable		$ 8,887

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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.

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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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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.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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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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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. 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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Amount by which current fair value of underlying stock exceeds exercise price of 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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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 price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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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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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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