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Significant accounting policies - Property, plant and equipment (Details)	12 Months Ended
	Dec. 31, 2017
Heavy equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5000 hours
Heavy equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	70000 hours
Major component parts in use | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	1000 hours
Major component parts in use | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	36000 hours
Other equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Other equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Licensed motor vehicles | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Licensed motor vehicles | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Office and computer equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	4 years
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	25 years

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- Definition

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.

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No definition available.

+ Details

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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