financial instruments - Market risks (Details) - CAD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Interest rate risk
Market risks
Percentage of reasonably possible changes in market risk variable	0.25%	0.25%
Other comprehensive income sensitivity to increase in risk variable	$ 4	$ 4
Comprehensive income sensitivity to increase in risk variable	4	4
Other comprehensive income sensitivity to decrease in risk variable	(4)	(3)
Comprehensive income sensitivity to decrease in risk variable	(4)	(3)
U.S. Dollars \| Currency risk
Market risks
Other comprehensive income sensitivity to increase in risk variable	(55)	(17)
Comprehensive income sensitivity to increase in risk variable	(55)	(17)
Other comprehensive income sensitivity to decrease in risk variable	55	17
Comprehensive income sensitivity to decrease in risk variable	$ 55	$ 17
Equity contributed, Common shares
Market risks
Data period for share price volatility	6 months	6 months
Share price volatility for sensitivity analysis (as a per cent)	12.50%	7.70%
Equity contributed, Common shares \| Other market risk
Market risks
Percentage of reasonably possible changes in market risk variable	25.00%	25.00%
Net income sensitivity to increase in risk variable	$ (4)	$ (15)
Other comprehensive income sensitivity to increase in risk variable	1	20
Comprehensive income sensitivity to increase in risk variable	(3)	5
Net income sensitivity to decrease in risk variable	19	23
Other comprehensive income sensitivity to decrease in risk variable	(1)	(20)
Comprehensive income sensitivity to decrease in risk variable	$ 18	$ 3

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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The increase (decrease) in comprehensive income, as a result of an decrease in the relevant risk variable that was reasonably possible at the end of the reporting period.

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The increase (decrease) in comprehensive income, as a result of an increase in the relevant risk variable that was reasonably possible at the end of the reporting period.

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The increase (decrease) in net income, as a result of an decrease in the relevant risk variable that was reasonably possible at the end of the reporting period.

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The increase (decrease) in net income, as a result of an increase in the relevant risk variable that was reasonably possible at the end of the reporting period.

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The increase (decrease) in other comprehensive income, as a result of an decrease in the relevant risk variable that was reasonably possible at the end of the reporting period.

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The increase (decrease) in other comprehensive income, as a result of an increase in the relevant risk variable that was reasonably possible at the end of the reporting period.

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- Definition

The reasonably possible percentage of change in the relevant risk variable used to determine the financial impact in a sensitivity analysis.

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Prior period for which data is used to calculate the volatility of the entity's share price as an assumption in a sensitivity analysis.

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Volatility in share price used as an assumption in a sensitivity analysis.

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