share-based compensation - Employee share purchase plan (Details) - Employee share purchase plan - CAD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
share-based compensation
Dividend reinvestment in Common Shares	$ 9	$ 8	$ 17	$ 15
Purchase plan discount			0.00%	0.00%

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