Share-based compensation plans - Option Activity (Details)	6 Months Ended
Jun. 30, 2021 $ / shares shares
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares \| shares	1,165,000.0
Granted, shares \| shares	0
Forfeited, shares \| shares	(184,700)
Exercised, shares \| shares	(33,800)
Ending balance, shares \| shares	946,500
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) \| $ / shares	$ 11.17
Granted (dollars per share) \| $ / shares	0
Forfeited (dollars per share) \| $ / shares	15.23
Exercised (dollars per share) \| $ / shares	14.72
Ending balance (dollars per share) \| $ / shares	$ 10.25
Time Based Vesting Option
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares \| shares	207,800
Granted, shares \| shares	0
Forfeited, shares \| shares	0
Exercised, shares \| shares	0
Ending balance, shares \| shares	207,800
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) \| $ / shares	$ 21.38
Granted (dollars per share) \| $ / shares	0
Forfeited (dollars per share) \| $ / shares	0
Exercised (dollars per share) \| $ / shares	0
Ending balance (dollars per share) \| $ / shares	$ 21.38

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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- Definition

Number of options outstanding, including both vested and non-vested options.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

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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- Definition

Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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- Definition

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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- Definition

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

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