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Investments in affiliates and joint ventures - Narrative (Details) - Variable Interest Entity, Not Primary Beneficiary	12 Months Ended
	Dec. 31, 2018
Dene North Site Services Partnership
Variable Interest Entity [Line Items]
NL Partnership Interest in VIEs	49.00%
Mikisew North American Limited Partnership
Variable Interest Entity [Line Items]
NL Partnership Interest in VIEs	49.00%

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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.

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- Definition

Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=SL6759159-111685

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