Pensions and Other Postretirement Benefits (Rollforward of Fair Value of Investments Classified as Level 3) (Details) - CAD CAD in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Fair Value of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	CAD 17,831
Fair value of plan assets at end of year	18,564	CAD 17,831
Real Estate [Member]
Defined Benefit Plan Fair Value of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	383
Fair value of plan assets at end of year	410	383
Oil and Gas [Member]
Defined Benefit Plan Fair Value of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	1,076
Fair value of plan assets at end of year	1,120	1,076
Fair Value Level 3 [Member]
Defined Benefit Plan Fair Value of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	1,046	1,097
Actual return relating to assets still held at the reporting date	107	(9)
Purchases		1
Sales		0
Disbursements	(52)	(43)
Fair value of plan assets at end of year	1,101	1,046
Fair Value Level 3 [Member] \| Real Estate [Member]
Defined Benefit Plan Fair Value of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	324	331
Actual return relating to assets still held at the reporting date	19	15
Purchases		1
Disbursements	(11)	(23)
Fair value of plan assets at end of year	332	324
Fair Value Level 3 [Member] \| Oil and Gas [Member]
Defined Benefit Plan Fair Value of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	722	766
Actual return relating to assets still held at the reporting date	88	(24)
Purchases		0
Disbursements	(41)	(20)
Fair value of plan assets at end of year	CAD 769	CAD 722

Amount of increase in plan assets from purchases of investments.

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- Definition

Amount of decrease in plan assets from sales of investments.

+ References

No definition available.

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- Definition

Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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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.

+ References

No definition available.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6749337

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6520858

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- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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