Share capital - Common shares and treasury shares (Details) - shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares issued, beginning of period	93,639,610	107,227,347	106,501,299
Treasury Stock, Shares, Retired	0	(14,256,043)	0
Stock Issued During Period, Shares, Conversion of Convertible Securities	0	361,556	0
Common shares issued, end of period	94,225,498	93,639,610	107,227,347
Balance, end of year	0	0	(3,944,920)
Common share, shares outstanding (in shares)	94,225,498	93,639,610	103,282,427
Stock Issued During Period, Shares, Conversion of Convertible Securities, Gross of Adjustments		1,156,184
Stock Issued During Period, Shares, Conversion of Convertible Securities, Adjustments		794,628
Management and director options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	187,678	0	150,802
Restricted share awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	366,453	50,644	(35,011)
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	31,757	256,106	610,257

Stock Issued During Period, Shares, Conversion of Convertible Securities, Adjustments

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Stock Issued During Period, Shares, Conversion of Convertible Securities, Gross of Adjustments

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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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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Number of shares issued during the period as a result of the conversion of convertible securities.

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Gross number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP). Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.

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Number of shares of common and preferred stock retired from treasury during the period.

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