Major Commitments and Contingencies (Guarantees and Indemnifications) (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Guarantee Obligations [Member]
Guarantees and indemnifications
Liability current carrying value	$ 0
Indemnification Agreement [Member]
Guarantees and indemnifications
Liability current carrying value	$ 0
Fair Value Guarantee [Member]
Guarantees and indemnifications
Maturity of guarantee instruments	between 2019 and 2024
Maximum potential liability under guarantees	$ 130	$ 141
Guarantee obligations, recourse provisions	There are no recourse provisions to recover any amounts from third parties
Performance Guarantee [Member]
Guarantees and indemnifications
Maturity of guarantee instruments	between 2019 and 2020
Performance Guarantee [Member] \| Committed Bilateral Letter of Credit Facilites [Member]
Guarantees and indemnifications
Maximum potential liability under guarantees	$ 707	697
Outstanding letters of credit	410	394
Performance Guarantee [Member] \| Uncommitted Bilateral Letter of Credit Facilities [Member]
Guarantees and indemnifications
Outstanding letters of credit	137	136
Performance Guarantee [Member] \| Surety and Other Bond [Member]
Guarantees and indemnifications
Liability current carrying value	160	167
Other Employee Benefit Liabilities and Workers' Compensation [Member]
Guarantees and indemnifications
Maximum potential liability under guarantees	659	648
Other Liabilities [Member]
Guarantees and indemnifications
Maximum potential liability under guarantees	$ 48	$ 49

No definition available.

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

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

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

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

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

- Definition

Describes the nature of any recourse provisions that would enable the guarantor to recover from third parties any of the amounts paid under the guarantee (or each group of similar guarantees).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsRecourseProvisions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Describe the approximate term of the guarantee or each group of similar guarantees.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Details

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- Details

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- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_MarketValueGuaranteeMember
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Data Type:	na
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- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_PerformanceGuaranteeMember
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- Details

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- Details

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- Details

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- Details

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- Details

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