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Investments in affiliates and joint ventures - Narrative (Details) | 12 Months Ended |
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Dec. 31, 2019 | Nov. 01, 2018
Nuna East Ltd. |  |
Variable Interest Entity [Line Items] |  |
Ownership percentage | 37.25% | 37.25%
Nuna West Mining Ltd. |  |
Variable Interest Entity [Line Items] |  |
Ownership percentage | 49.00% | 49.00%
Nuna Pang Contracting Ltd. |  |
Variable Interest Entity [Line Items] |  |
Ownership percentage | 37.25% | 37.25%
Nuna Logistics Partnership |  |
Variable Interest Entity [Line Items] |  |
Ownership percentage | 49.00% | 49.00%
Dene North Site Services Partnership | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
NL Partnership Interest in VIEs | 49.00% |
Mikisew North American Limited Partnership [Member] | Variable Interest Entity, Not Primary Beneficiary |  |
Variable Interest Entity [Line Items] |  |
NL Partnership Interest in VIEs | 49.00% |

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