Share-based compensation plans - Option Activity (Details)	9 Months Ended
Sep. 30, 2019 $ / shares shares
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares \| shares	1,287,000.0
Granted, shares \| shares	0
Forfeited, shares \| shares	0
Exercised, shares \| shares	0
Ending balance, shares \| shares	1,287,000.0
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) \| $ / shares	$ 10.88
Granted (dollars per share) \| $ / shares	0
Forfeited (dollars per share) \| $ / shares	0
Exercised (dollars per share) \| $ / shares	0
Ending balance (dollars per share) \| $ / shares	$ 14.00
Time Based Vesting Option
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares \| shares	2,700
Granted, shares \| shares	138,700
Forfeited, shares \| shares	(14,400)
Exercised, shares \| shares	0
Ending balance, shares \| shares	127,000.0
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) \| $ / shares	$ 21.09
Granted (dollars per share) \| $ / shares	21.58
Forfeited (dollars per share) \| $ / shares	21.60
Exercised (dollars per share) \| $ / shares	0
Ending balance (dollars per share) \| $ / shares	$ 21.56

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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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
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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

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