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Decommissioning provision (Narrative) (Details) - Decommissioning Provision [Member] $ in Millions | 12 Months Ended
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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

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