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Financial instruments and risk management - Risk management (Details) - CAD ($) $ in Millions	12 Months Ended
	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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Debt Instrument, Basis Spread on Variable Rate, Adjustment

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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