Resulting performance vesting factor from the return on investment level obtained.

No definition available.

Name:	cni_PerformanceVestingFactor
Namespace Prefix:	cni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of units to be settled from the Share Trusts in the next fiscal year.

+ References

No definition available.

+ Details

Name:	cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsSettledByUnitsFromShareTrusts
Namespace Prefix:	cni_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The payout to be paid in the first quarter of the next fiscal year due to conditions of cash settled performance share units being met.

+ References

No definition available.

+ Details

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

- Definition

The number of units of cash settled stock-based compensation awards modified in 2014 to settle in common shares of the Company.

+ References

No definition available.

+ Details

Name:	cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnitsModified
Namespace Prefix:	cni_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

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

- Definition

An excess of the fair value of the modified award over the fair value of the award immediately before the modification.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The number of employees affected by the modification of an equity-based compensation plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationNumberOfEmployeesAffected
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	cni_ShareBasedCompensationArrangementsByShareBasedPaymentAwardTypeAxis=cni_PerformanceShareUnitROICMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cni_ShareBasedCompensationArrangementsByShareBasedPaymentAwardTypeAxis=cni_PerformanceShareUnitTotalShareholderReturnMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=cni_EquitySettledAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=cni_GrantedYear2013Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=cni_CashSettledAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=cni_GrantedYear2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=cni_ShareUnitsPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-based compensation (Narrative) (Details 2) shares in Millions, CAD in Millions	12 Months Ended
	Dec. 31, 2015 CAD shares	Dec. 31, 2014 CAD shares	Dec. 31, 2013 CAD shares
PSUs-ROIC
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees affected by modification to stock-based compensation plan		133
PSUs-ROIC \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance vesting factor	0.00%	0.00%
PSUs-ROIC \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance vesting factor	200.00%	150.00%
PSUs-TSR \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance vesting factor	0.00%
PSUs-TSR \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance vesting factor	200.00%
Equity settled awards \| PSUs-ROIC
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Settlement from the Share Trusts, next fiscal year \| shares	0.6
Equity settled awards \| PSUs-ROIC \| 2013 grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance vesting factor	150.00%
Fair value of awards vested during the period \| CAD	CAD 48
Cash settled awards \| PSUs-ROIC
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of awards vested during the period \| CAD		CAD 106	CAD 80
Units granted during the period PSUs (in shares) \| shares		0.8	0.8
Cash settled awards \| PSUs-ROIC \| 2013 grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance vesting factor	150.00%
Fair value of awards vested during the period \| CAD	CAD 39
Cash settled awards \| PSUs-ROIC \| 2014 grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash settled awards modified to equity settled awards \| shares		0.4
Share Units Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental compensation cost from modification \| CAD	CAD 0
Share Units Plan \| PSUs-ROIC
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The number of years in the plan period over which the units will vest	3 years
Share Units Plan \| PSUs-TSR
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The number of years in the plan period over which the units will vest	3 years
Share Units Plan \| Cash settled awards \| PSUs-ROIC
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payout related to cash settled PSUs in the first quarter of next fiscal year \| CAD	CAD 39
Units granted during the period PSUs (in shares) \| shares	0.0
Share Units Plan \| Cash settled awards \| PSUs-ROIC \| 2013 grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash settled awards modified to equity settled awards \| shares		0.5