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Accumulated Other Comprehensive Income (Components of Accumulated Other Comprehensive Income) (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (165)	$ 135
Other comprehensive income (loss) before reclassifications	504	(347)
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	(1)	47
Net current period other comprehensive income (loss)	503	(300)
Ending Balance	338	(165)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(3)	(21)
Other comprehensive income (loss) before reclassifications	2	10
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	(6)	8
Net current period other comprehensive income (loss)	(4)	18
Ending Balance	(7)	(3)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(243)	(283)
Other comprehensive income (loss) before reclassifications	0	0
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	9	40
Net current period other comprehensive income (loss)	9	40
Ending Balance	(234)	(243)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	48	(49)
Other comprehensive income (loss) before reclassifications	(122)	97
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	0	0
Net current period other comprehensive income (loss)	(122)	97
Ending Balance	(74)	48
Accumulated Net Available For Sale Securities Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	3	(1)
Other comprehensive income (loss) before reclassifications	0	5
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	(4)	(1)
Net current period other comprehensive income (loss)	(4)	4
Ending Balance	(1)	3
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	30	489
Other comprehensive income (loss) before reclassifications	624	(459)
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	0	0
Net current period other comprehensive income (loss)	624	(459)
Ending Balance	$ 654	$ 30

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Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, attributable to parent. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84158357&loc=d3e1436-108581

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84161450&loc=SL4573702-111684

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4569616-111683

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
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-Section 45
-Paragraph 20
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Amount before tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

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