Financial instruments and fair value disclosures - Trade receivables and allowance for doubtful accounts (Details) - Trade accounts receivables - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Disclosure of financial assets [line items]
Trade receivables, net of allowance for doubtful accounts	$ 273.1	$ 175.9
Cost
Disclosure of financial assets [line items]
Trade receivables, net of allowance for doubtful accounts	282.8	180.9
Less: allowance for doubtful accounts
Disclosure of financial assets [line items]
Trade receivables, net of allowance for doubtful accounts	$ (9.7)	$ (5.0)

- Definition

The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Financial instruments, class [member]; Financial liabilities]

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