Share-based payments (Summary of Stock Option Activity) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Share-based payments
Outstanding beginning balance, Common shares under option	2,797,189
Granted, Common shares under option	642,380
Exercised, Common shares under option	(247,446)
Forfeited, Common shares under option	(22,016)
Expired, Common shares under option	(1,201)
Outstanding ending balance, Common shares under option	3,168,906	2,797,189
Exercisable, Common shares under option	1,790,002
Outstanding beginning balance, Weighted average exercise price (per share)	$ 29.05
Granted, Weighted average exercise price (per share)	40.64
Exercised, Weighted average exercise price (per share)	28.51
Forfeited, Weighted average exercise price (per share)	30.11
Expired, Weighted average exercise price (per share)	21.82
Outstanding ending balance, Weighted average exercise price (per share)	31.44	$ 29.05
Exercisable, Weighted average exercise price (per share)	$ 27.68
Outstanding, Weighted average remaining contractual life (in years)	7 years 7 months 6 days	7 years 1 month 6 days
Exercisable, Weighted average remaining contractual life (in years)	6 years 6 months
Outstanding beginning balance, Aggregate intrinsic value	$ 38,874
Exercised, Aggregate intrinsic value	3,536
Outstanding ending balance, Aggregate intrinsic value	13,039	$ 38,874
Exercisable, Aggregate intrinsic value	$ 11,638

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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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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.

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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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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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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 of the plan that expired.

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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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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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