Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ Details

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

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=117329964&loc=d3e12355-112629

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=117329964&loc=d3e12317-112629

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

+ Details

Name:	us-gaap_RepaymentsOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandFiftyThirtyYearNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandTwentyNineTenYearNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandFortyNineNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandFortyNineThirtyYearNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_Cnr2019NotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandTwentyEightTenYearNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandFortyEightNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_Cnr2018Notes20YrMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_Cnr2018Notes10YrMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandTwentyTenYearNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cni_CNRTwoThousandFortyEightThirtyYearNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt (Notes and debentures) (Details)	Nov. 01, 2019 CAD ($)	Feb. 08, 2019 CAD ($)	Nov. 07, 2018 CAD ($)	Aug. 30, 2018 CAD ($)	Aug. 30, 2018 USD ($)	Jul. 31, 2018 CAD ($)	Jul. 15, 2018 CAD ($)	Jul. 15, 2018 USD ($)	May 15, 2018 CAD ($)	May 15, 2018 USD ($)	Feb. 06, 2018 CAD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)	Nov. 07, 2018 USD ($)	Feb. 06, 2018 USD ($)
Debt Instrument [Line Items]
Proceeds from debt issuance	$ 443,000,000	$ 790,000,000				$ 787,000,000					$ 1,106,000,000
Canadian National series, 3.05% 30-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount	$ 450,000,000
Interest rate	3.05%											3.05%	3.05%
Total debt, gross												$ 450,000,000		$ 0
Canadian National series, 3.00% 10-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount		$ 350,000,000
Interest rate		3.00%										3.00%	3.00%
Total debt, gross												$ 350,000,000		0
Canadian National series, 3.60% 30-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount		$ 450,000,000
Interest rate		3.60%										3.60%	3.60%
Total debt, gross												$ 450,000,000		0
Canadian National series, 4.45% 30-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount			$ 854,000,000													$ 650,000,000
Interest rate			4.45%									4.45%	4.45%			4.45%
Proceeds from debt issuance			$ 845,000,000
Total debt, gross												$ 844,000,000	$ 650,000,000	886,000,000	$ 650,000,000
Canadian National series, 5/55% 10-year notes [Member]
Debt Instrument [Line Items]
Interest rate					5.55%
Total debt, gross					$ 550,000,000
Repayment of debt				$ 720,000,000	558,000,000
Loss on debt repayment				$ 10,000,000	$ 8,000,000
Canadian National series, 3.20% 10-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount						$ 350,000,000
Interest rate						3.20%						3.20%	3.20%
Total debt, gross												$ 350,000,000		350,000,000
Canadian National series, 3.60% 30-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount						$ 450,000,000
Interest rate						3.60%						3.60%	3.60%
Total debt, gross												$ 450,000,000		450,000,000
Canadian National series, 6.80% 20-year notes [Member]
Debt Instrument [Line Items]
Interest rate							6.80%	6.80%
Repayment of debt							$ 264,000,000	$ 200,000,000
Canadian National series, 5.55% 10-year notes [Member]
Debt Instrument [Line Items]
Interest rate									5.55%	5.55%
Repayment of debt									$ 415,000,000	$ 325,000,000
Canadian National series, 2.40% 2-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount											$ 374,000,000						$ 300,000,000
Interest rate											2.40%	2.40%	2.40%				2.40%
Total debt, gross												$ 390,000,000	$ 300,000,000	409,000,000	300,000,000
Canadian National series, 3.65% 30-year notes [Member]
Debt Instrument [Line Items]
Debt issued, face amount											$ 749,000,000						$ 600,000,000
Interest rate											3.65%	3.65%	3.65%				3.65%
Total debt, gross												$ 779,000,000	$ 600,000,000	$ 818,000,000	$ 600,000,000