Shareholders Equity - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		11 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	May 10, 2017
Common Shares repurchased	$ 318
Current Normal Course Issuer Bid (NCIB) [Member]
Share repurchase expiration date	May 14, 2018
Common Shares repurchased (in shares)	1,435,700		3,300,000
Common Shares repurchased	$ 318		$ 699
Current Normal Course Issuer Bid (NCIB) [Member] | Maximum [Member]
Common shares authorized to be repurchased (in shares)				4,380,000
Normal Course Issuer Bid (NCIB) [Member]
Common Shares repurchased (in shares)		0
Common Shares repurchased		$ 0