Shareholders' Equity - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended		8 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Oct. 19, 2018	May 10, 2017
Common Shares repurchased (shares)	900,000	1,200,000	1,600,000	2,500,000
Common Shares repurchased	$ 296	$ 241	$ 481	$ 559
Current Normal Course Issuer Bid (NCIB) [Member]
Common shares authorized to be repurchased (in shares)						5,680,000
Common Shares repurchased (shares)	956,243		1,663,921		3,850,000
Common Shares repurchased	$ 296		$ 481		$ 1,049
2017 Normal Course Issuer Bid (NCIB) [Member]
Common shares authorized to be repurchased (in shares)							4,380,000
Common Shares repurchased (shares)		1,060,262		2,495,962
Common Shares repurchased		$ 241		$ 559