Pensions and Other Postretirement Benefits (Rollforward of Fair Value of Investments Classified as Level 3) (Details) - Pensions [Member] - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	$ 18,424	$ 17,244
Fair value of plan assets at end of year	19,723	18,424
Real estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	435
Fair value of plan assets at end of year	381	435
Oil and gas [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	901
Fair value of plan assets at end of year	649	901
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	1,036	1,049
Actual return relating to assets still held at the reporting date	(242)	20
Purchases	6	3
Sales	(1)	(1)
Disbursements	(16)	(35)
Fair value of plan assets at end of year	783	1,036
Fair Value, Inputs, Level 3 [Member] \| Real estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	329	321
Actual return relating to assets still held at the reporting date	(54)	13
Purchases	6	3
Sales	(1)	(1)
Disbursements	(1)	(7)
Fair value of plan assets at end of year	279	329
Fair Value, Inputs, Level 3 [Member] \| Oil and gas [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	707	728
Actual return relating to assets still held at the reporting date	(188)	7
Purchases	0	0
Sales	0	0
Disbursements	(15)	(28)
Fair value of plan assets at end of year	$ 504	$ 707

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- Definition

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.

+ References

No definition available.

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- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
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- Definition

Amount, measured using unobservable input, of decrease in plan asset of defined benefit plan from sale.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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- Definition

Amount, measured using unobservable input, of decrease in plan asset of defined benefit plan from settlement of trade associated with underlying investment.

+ References

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- Definition

Amount, measured using unobservable input, of increase in plan asset of defined benefit plan from purchase.

+ References

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