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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, 2019 CAD ($)
Canada | Capital loss
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 80
Unrecognized Amounts	(80)
Net Tax Credit Carryforwards	$ 0
Expiration period	Indefinite
Canada | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 1,131
Unrecognized Amounts	(554)
Net Tax Credit Carryforwards	$ 577
Expiration period	2027-2039
United States | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 2,394
Unrecognized Amounts	0
Net Tax Credit Carryforwards	$ 2,394
Expiration period	2024-2037
United States | Tax credit
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 311
Unrecognized Amounts	0
Net Tax Credit Carryforwards	$ 311
Expiration period	2020-Indefinite
State | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 1,174
Unrecognized Amounts	0
Net Tax Credit Carryforwards	$ 1,174
Expiration period	2024-2039
Other | NOL
Composition of taxes on income from continuing operations [Line items]
Gross Tax Carryforwards	$ 38
Unrecognized Amounts	(38)
Net Tax Credit Carryforwards	$ 0
Expiration period	2020-2026

X

- Definition

The description of the expiration period applicable to the net operating loss, capital loss and tax credit carryforwards.

+ References

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- Definition

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

+ 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)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

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