Pensions and other postretirement benefits (Details 4) - CAD CAD in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Fair Value Of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	CAD 17,917
Fair value of plan assets at end of year	17,831	CAD 17,917
Real Estate [Member]
Defined Benefit Plan Fair Value Of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	357
Fair value of plan assets at end of year	383	357
Oil And Gas [Member]
Defined Benefit Plan Fair Value Of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	1,012
Fair value of plan assets at end of year	1,076	1,012
Fair Value Level 3 [Member]
Defined Benefit Plan Fair Value Of Investments Disclosure [Line Items]
Fair value of plan assets at beginning of year	1,097	1,325
Actual return relating to assets still held at the reporting date	(9)	(179)
Purchases	1	23
Sales		(3)
Disbursements	(43)	(69)
Fair value of plan assets at end of year	1,046	1,097
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	331	317
Actual return relating to assets still held at the reporting date	15	34
Purchases	1	23
Sales		(3)
Disbursements	(23)	(40)
Fair value of plan assets at end of year	324	331
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	766	1,008
Actual return relating to assets still held at the reporting date	(24)	(213)
Purchases	0	0
Sales		0
Disbursements	(20)	(29)
Fair value of plan assets at end of year	CAD 722	CAD 766

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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