Share-based compensation plans - Company's SARs (Details) - Stock Appreciation Rights (SARs)	12 Months Ended
Dec. 31, 2019 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning Balance (in shares)	1,832,358
Vested (in shares)	0
Granted (in shares)	543,278
Exercised (in shares)	(664,797)
Forfeited/Expired (in shares)	(148,716)
Ending Balance (in shares)	1,562,123
Exercisable (in shares)	655,110
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Beginning Balance, weighted-average exercise price (usd per share) \| $ / shares	$ 47.41
Granted, weighted-average exercise price (usd per share) \| $ / shares	60.76
Exercised, weighted-average exercise price (usd per share) \| $ / shares	43.92
Forfeited/Expired, weighted average exercise price (usd per share) \| $ / shares	53.36
Ending Balance, weighted average exercise price (usd per share) \| $ / shares	52.95
Exercisable, weighted-average exercise price (usd per share) \| $ / shares	$ 47.69
Nonvested
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning Balance (in shares)	1,119,602
Vested (in shares)	620,026
Granted (in shares)	543,278
Exercised (in shares)	0
Forfeited/Expired (in shares)	(135,841)
Ending Balance (in shares)	907,013
Exercisable (in shares)	0
Vested
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning Balance (in shares)	712,756
Vested (in shares)	620,026
Granted (in shares)	0
Exercised (in shares)	(664,797)
Forfeited/Expired (in shares)	(12,875)
Ending Balance (in shares)	655,110
Exercisable (in shares)	655,110

Share based compensation arrangement by share based payment award options vested in period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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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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- Definition

Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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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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- Definition

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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