Shareholders' Equity - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended		11 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Oct. 19, 2018	May 10, 2017
Number of Common Shares repurchased	1,600,000		3,200,000	2,500,000
Amount of repurchase (in millions)	$ 471		$ 952	$ 559
Current Normal Course Issuer Bid (NCIB) [Member]
Common Shares authorized to be repurchased						5,680,000
Number of Common Shares repurchased	1,519,540		3,183,461		5,370,000
Amount of repurchase (in millions)	$ 471		$ 952		$ 1,520
2017 Normal Course Issuer Bid (NCIB) [Member]
Common Shares authorized to be repurchased							4,380,000
Number of Common Shares repurchased		0		2,495,962
Amount of repurchase (in millions)		$ 0		$ 559