Provisions and other non-current liabilities - Other risks and contingent liabilities (Details) - Contingent liabilities - legal proceedings - FERC	Apr. 28, 2016 employee	Dec. 31, 2018 company
Disclosure of contingent liabilities
Number of former employees issued order to show cause \| employee	2
Number of companies involved in class action lawsuit \| company		3

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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