Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_StockRepurchasedDuringPeriodValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Shareholders Equity - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		5 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Oct. 19, 2018	May 10, 2017
Common Shares repurchased	$ 185	$ 318
Current Normal Course Issuer Bid (NCIB) [Member]
Common shares authorized to be repurchased (in shares)				5,680,000
Share repurchase expiration date	Oct. 23, 2019
Common Shares repurchased (shares)	707,678		2,890,000
Common Shares repurchased	$ 185		$ 753
2017 Normal Course Issuer Bid (NCIB) [Member]
Common shares authorized to be repurchased (in shares)					4,380,000
Share repurchase expiration date		May 14, 2018
Common Shares repurchased (shares)		1,435,700
Common Shares repurchased		$ 318