Shareholders' equity - Acquired shares (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Class B common shares
Class of Stock [Line Items]
Reduction due to class B common shares owned by a subsidiary (in shares)	18,044,478	18,044,478

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.

No definition available.

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