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Cumulative Preferred Stock (Details) - CAD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Preferred Stock, Shares Outstanding	29,000,000	29,000,000
Proceeds From Issuance Of Preferred Stock And Preference Stock Net	$ 709	$ 709
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared		$ 0.6388
Preferred Stock, Redemption Price Per Share	$ 25
Preferred Stock, Shares Outstanding	3,864,636	3,864,636
Proceeds From Issuance Of Preferred Stock And Preference Stock Net		$ 95
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared	$ 0.6032	$ 0.5724
Preferred Stock, Redemption Price Per Share	$ 25
Preferred Stock, Shares Outstanding	2,135,364	2,135,364
Proceeds From Issuance Of Preferred Stock And Preference Stock Net		$ 52
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared		$ 1.025
Preferred Stock, Redemption Price Per Share	$ 25
Preferred Stock, Shares Outstanding	10,000,000	10,000,000
Proceeds From Issuance Of Preferred Stock And Preference Stock Net		$ 245
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared		$ 1.125
Preferred Stock, Redemption Price Per Share	$ 26
Preferred Stock, Shares Outstanding	5,000,000	5,000,000
Proceeds From Issuance Of Preferred Stock And Preference Stock Net		$ 122
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared		$ 1.0625
Preferred Stock, Redemption Price Per Share	$ 25
Preferred Stock, Shares Outstanding	8,000,000	8,000,000
Proceeds From Issuance Of Preferred Stock And Preference Stock Net		$ 195

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The price per share at which the preferred stock of an entity that has priority over common stock in the distribution of dividends and in the event of liquidation of the entity is redeemed or may be called at. The redemption features of this preferred stock are solely within the control of the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21564-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21488-112644

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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

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