Other Income and Charges (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Income and Expenses [Abstract]
Foreign exchange loss (gain) on long-term debt	$ 44	$ (67)	$ 93	$ (95)
Other foreign exchange losses (gains)	4	0	3	(1)
Insurance recovery of legal settlement	0	(10)	0	(10)
Charge on hedge roll and de-designation	0	13	0	13
Other	4	3	7	4
Total other income and charges	$ 52	$ (61)	$ 103	$ (89)