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Decommissioning provision (Narrative) (Details) - Decommissioning Provision [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Disclosure of other provisions [line items]
Estimated undiscounted amount of decommissioning provision	$ 5.6
Bottom of range [member]
Disclosure of other provisions [line items]
Risk free rate used to discount decommissioning provision	2.30%
Term used to estimate decommissioning provision	2 years
Top of range [member]
Disclosure of other provisions [line items]
Risk free rate used to discount decommissioning provision	7.60%
Term used to estimate decommissioning provision	12 years