Other liabilities and deferred credits (Details) (CAD) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Other Liabilities And Deferred Credits [Line Items]
Personal injury and other claims provisions, net of current portion	250	271
Stock-based compensation liability, net of current portion	91	240
Environmental provisions, net of current portion	69	78
Deferred credits and other	294	226
Total other liabilities and deferred credits	704us-gaap_OtherLiabilitiesNoncurrent	815us-gaap_OtherLiabilitiesNoncurrent

Name:	cni_OtherLiabilitiesAndDeferredCreditsLineItems
Namespace Prefix:	cni_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This element reflects the non-current estimated ultimate cost of settling claims relating to workers compensation, personal injury and legal and other claims that have occurred on or before the balance sheet date.

+ References

No definition available.

+ Details

Name:	cni_PersonalInjuryAndOtherClaimsProvisionNonCurrent
Namespace Prefix:	cni_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable after twelve months or beyond the next operating cycle if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.24)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 24

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 30

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 30

-Section 50

-Paragraph 10

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

+ Details

Name:	us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements payable after one year (or the operating cycle, if longer).

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCompensationSharebasedArrangementsLiabilityClassifiedNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of unearned revenue or income, not otherwise specified in the taxonomy, which is expected to be taken into income after one year or beyond the normal operating cycle, if longer and noncurrent obligations not separately disclosed in the balance sheet (other liabilities, noncurrent).

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCreditsAndOtherLiabilitiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate carrying amount, as of the balance sheet date, of noncurrent obligations not separately disclosed in the balance sheet. Noncurrent liabilities are expected to be paid after one year (or the normal operating cycle, if longer).

+ References

+ Details

Name:	us-gaap_OtherLiabilitiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant