Share capital - Common shares (Details) - shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares issued, beginning of year	94,225,498	93,639,610	107,227,347
Treasury Stock, Shares, Retired	0	0	(14,256,043)
Stock Issued During Period, Shares, Conversion of Convertible Securities	0	0	361,556
Common shares issued, end of year	95,582,733	94,225,498	93,639,610
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	0	187,678	0
Restricted share awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	1,012,939	366,453	50,644
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	344,296	31,757	256,106

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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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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Number, before forfeiture, of shares issued under share-based payment arrangement. Excludes employee stock ownership plan (ESOP).

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

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