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Financial instruments and risk management - Risk management (Details) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Customer 1 | Accounts receivable and contract assets | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 33.00% | 42.00%
Customer 2 | Accounts receivable and contract assets | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 16.00% | 20.00%
Customer 3 | Accounts receivable and contract assets | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 12.00% | 10.00%
Customer 4 | Accounts receivable and contract assets | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 7.00% | 10.00%
Customer 5 | Accounts receivable and contract assets | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 4.00% | 12.00%
Credit facilities |  |
Concentration Risk [Line Items] |  |
Outstanding balance, long-term debt | $ 194.9 | $ 32.0
Corresponding change in annual interest expense | $ 1.9 |
Basis on variable rate, adjustment | 1.00% |

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