Stock-based compensation - Summary of Additional Information Relating to Service-based Options (Details) - Service-based Stock Options - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 590	$ 2,308	$ 3,219	$ 2,435
Intrinsic value of options exercised	$ 0	$ 3,580	$ 2,890	$ 12,841
Weighted-average grant date fair value of options granted (in USD per share)	$ 3.12	$ 5.55	$ 6.81	$ 9.51

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

+ References

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-Publisher FASB
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-Section 50
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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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