Major Commitments and Contingencies (Guarantees and Indemnifications) (Details) - CAD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Guarantee Obligations [Member]
Guarantees and indemnifications
Liability current carrying value	$ 0
Indemnification Agreement [Member]
Guarantees and indemnifications
Liability current carrying value	$ 0
Performance Guarantee [Member]
Guarantees and indemnifications
Maturity of guarantee instruments	between 2021 and 2025
Performance Guarantee [Member] \| Committed Bilateral Letter of Credit Facilites [Member]
Guarantees and indemnifications
Outstanding letters of credit	$ 421,000,000	$ 424,000,000
Maximum potential liability under guarantees	754,000,000	742,000,000
Performance Guarantee [Member] \| Uncommitted Bilateral Letter of Credit Facilities [Member]
Guarantees and indemnifications
Outstanding letters of credit	165,000,000	149,000,000
Performance Guarantee [Member] \| Surety and Other Bond [Member]
Guarantees and indemnifications
Liability current carrying value	168,000,000	169,000,000
Other Employee Benefit Liabilities and Workers' Compensation [Member]
Guarantees and indemnifications
Maximum potential liability under guarantees	688,000,000	681,000,000
Other Liabilities [Member]
Guarantees and indemnifications
Maximum potential liability under guarantees	$ 66,000,000	$ 61,000,000

No definition available.

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Namespace Prefix:	cni_
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- 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=121555522&loc=d3e12069-110248

+ Details

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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=121555522&loc=d3e12069-110248

+ Details

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

- 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=121555522&loc=d3e12069-110248

+ Details

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Balance Type:	na
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- 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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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- 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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- Details

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- Details

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- Details

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