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Financial instruments and risk management - Risk management (Details) - CAD CAD in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Customer A | Total revenues | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 44.00% | 47.00%
Customer B | Total revenues | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 26.00% | 25.00%
Customer C | Total revenues | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 17.00% | 21.00%
Customer 1 | Accounts receivable and unbilled revenue | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 42.00% | 42.00%
Customer 2 | Accounts receivable and unbilled revenue | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 20.00% | 22.00%
Customer 3 | Accounts receivable and unbilled revenue | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 10.00% | 7.00%
Customer 4 | Accounts receivable and unbilled revenue | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 10.00% | 13.00%
Customer 5 | Accounts receivable and unbilled revenue | Major customers |  |
Concentration Risk [Line Items] |  |
Concentration risk, percentage | 2.00% | 13.00%
Credit facilities |  |
Concentration Risk [Line Items] |  |
Outstanding balance, long-term debt | CAD 32.0 | CAD 39.6
Corresponding change in annual interest expense | CAD 0.3 |

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