Income Taxes - Narrative (Details) $ in Millions, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021 CAD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 CAD ($)	Jun. 30, 2021 CAD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 CAD ($)
Income Tax Disclosure [Abstract]
Effective tax rate	17.10%	17.10%	22.98%	19.50%	19.50%	26.38%
Effective tax rate, excluding discrete items	24.60%	24.60%	25.00%	24.60%	24.60%	25.00%
Foreign exchange (gain) loss on debt and lease liabilities	$ (52)		$ (86)	$ (85)		$ 129
Merger termination fee	845	$ 700	$ 0	845	$ 700	$ 0
Acquisition-related costs	$ 308			$ 344

Effective Income Tax Rate Reconciliation, Estimated Rate, Excluding Discrete Items, Percent

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- Definition

The aggregate foreign currency transaction gain or loss on long-term debt (both realized and unrealized) and lease liabilities included in determining net income for the reporting period net of hedging activity.

+ References

No definition available.

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- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e1043-128460

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- Definition

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

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- Definition

Gain (loss) related to the termination of a contract between the parties. The termination may be due to many causes including early termination of a lease by a lessee, a breach of contract by one party, or a failure to perform.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869

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