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Significant accounting policies - Revenue recognition (Details)	12 Months Ended
	Dec. 31, 2017
Minimum
Property, Plant and Equipment [Line Items]
Term of contract (less than one year)	1 year
Dene North Site Services
Property, Plant and Equipment [Line Items]
Undivided ownership interest, percentage	49.00%

X

- Definition

Jointly Owned Partnership, Proportionate Ownership Share

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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X

- Definition

Term of Contract

+ References

No definition available.

+ Details

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Balance Type:	na
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X

- 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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X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: