Subsequent Events (Details) - Subsequent Event [Member] CAD in Billions	12 Months Ended
Dec. 31, 2017 CAD
Subsequent events
Subsequent event date	Jan. 24, 2018
Subsequent event description	On January 24, 2018, the Company filed a preliminary shelf prospectus with Canadian securities regulators, pursuant to which CN may issue up to an aggregate amount of $6.0 billion of debt securities over a 25-month period. The final shelf prospectus and the corresponding U.S. registration statement are expected to be filed in early February 2018, and will replace CN’s existing shelf prospectus and registration statement that expire on February 6, 2018.
Maximum amount of debt securities issuance pursuant to shelf prospectus and registration statement	CAD 6.0

Maximum amount of debt securities that the entity may issue under the shelf prospectus filed with the Canadian securities regulators and registration statements filed with the SEC.

No definition available.

+ Details

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- Definition

Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 855
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6842918&loc=SL6314017-165662

+ Details

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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- Definition

Date the event occurred after the balance sheet date but before financial statements are issues or available to be issued, in CCYY-MM-DD format.

+ References

+ Details

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- Details

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