Stock-based compensation - Summary of Performance-based and Market-based Stock Option Activity and Related Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2020
Options outstanding
Exercised (in shares)	(62,450)
Weighted-average exercise price
Exercised (in USD per share)	$ 1.84
Closing stock price (in USD per share)		$ 9.84
Performance-based and Market-based Stock Options
Options outstanding
Beginning balance (in shares)	1,482,782	1,323,432
Exercised (in shares)		(53,100)
Canceled or forfeited (in shares)	(96,900)	(17,400)
Ending balance (in shares)	1,323,432	1,252,932
Weighted-average exercise price
Beginning balance (in USD per share)	$ 8.94	$ 7.96
Exercised (in USD per share)		2.40
Canceled or forfeited (in USD per share)	26.84	26.84
Ending balance (in USD per share)	$ 7.96	$ 7.97
Weighted-average remaining contractual life (in years), Outstanding	6 years	4 years 11 months 16 days
Aggregate intrinsic values, Outstanding	$ 8,646	$ 7,487
Options outstanding, Exercisable (in shares)		952,932
Weighted-average exercise price, Exercisable (in USD per share)		$ 1.98
Weighted-average remaining contractual life (in years), Exercisable		4 years 4 months 7 days
Aggregate intrinsic values, Exercisable		$ 7,487

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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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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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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Price of a single share of a number of saleable stocks of a company.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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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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