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Significant accounting policies - Property, plant and equipment (Details) | 12 Months Ended
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Dec. 31, 2018
Heavy equipment | Minimum |
Property, Plant and Equipment [Line Items] |
Property, plant and equipment, useful life | 3000 hours
Heavy equipment | Maximum |
Property, Plant and Equipment [Line Items] |
Property, plant and equipment, useful life | 120000 hours
Major component parts in use | Minimum |
Property, Plant and Equipment [Line Items] |
Property, plant and equipment, useful life | 3000 hours
Major component parts in use | Maximum |
Property, Plant and Equipment [Line Items] |
Property, plant and equipment, useful life | 50000 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 | 5 years
Furnishings, fixtures and facilities | Minimum |
Property, Plant and Equipment [Line Items] |
Property, plant and equipment, useful life | 10 years
Furnishings, fixtures and facilities | Maximum |
Property, Plant and Equipment [Line Items] |
Property, plant and equipment, useful life | 30 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 | 50 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. + References No definition available. + Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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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Namespace Prefix: | us-gaap_
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- Details
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Data Type: | na
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- Details
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Data Type: | na
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- Details
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- Details
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Data Type: | na
Balance Type: |
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- Details
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Data Type: | na
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- Details
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Data Type: | na
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- Details
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- Details
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Data Type: | na
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- Details
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