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Income Taxes (Net operating loss, capital loss and tax credit carryforwards and their expiration periods) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 CAD ($)
Canada | Capital loss
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 61
Unrecognized Amounts	(61)
Net Tax Credit Carryforwards	$ 0
Expiration period	Indefinite
Canada | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 1,370
Unrecognized Amounts	(619)
Net Tax Credit Carryforwards	$ 751
Expiration period	2027 - 2040
United States | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 1,412
Unrecognized Amounts	0
Net Tax Credit Carryforwards	$ 1,412
Expiration period	2030 - 2040
United States | Tax credit
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 319
Unrecognized Amounts	0
Net Tax Credit Carryforwards	$ 319
Expiration period	2025 - 2040
State | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 563
Unrecognized Amounts	0
Net Tax Credit Carryforwards	$ 563
Expiration period	2032 - 2040
Other | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 39
Unrecognized Amounts	(39)
Net Tax Credit Carryforwards	$ 0
Expiration period	2021 - 2027

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- Definition

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+ References

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- Definition

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+ References

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- Definition

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Amount of valuation allowance pertaining to the deferred tax asset representing potential future taxable deductions from tax credit carryforwards for which it is more likely than not that a tax benefit will not be realized.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
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