Assets Held For Sale (Details) - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Impairment loss, pre-tax	$ 486
Loss on assets held for sale	$ 363	$ 486	$ 0	$ 0
Assets held for sale		$ 90

Amount classified as property, plant and equipment attributable to disposal group held for sale or disposed of.

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Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant