Debt - Narrative (Details)				12 Months Ended
	Nov. 15, 2023 USD ($)	May 15, 2023 USD ($)	Apr. 19, 2023 USD ($) series	Dec. 31, 2023 USD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2021 CAD ($)	Dec. 31, 2023 CAD ($)	Jul. 12, 2023 USD ($)	Jul. 11, 2023 USD ($)	May 11, 2023 USD ($)	May 10, 2023 USD ($)	Apr. 24, 2023 USD ($) series	Dec. 31, 2022 CAD ($)	Feb. 21, 2012 USD ($)	Dec. 31, 2011 locomotive	Jun. 28, 2005 USD ($)
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 22,598,000,000						$ 19,771,000,000
Annual maturities and principal repayments in 2024									3,133,000,000
Annual maturities and principal repayments in 2025									933,000,000
Annual maturities and principal repayments in 2026									1,990,000,000
Annual maturities and principal repayments in 2027									7,000,000
Annual maturities and principal repayments in 2028									1,868,000,000
Annual maturities and principal repayments thereafter									15,202,000,000
Number of series of notes | series			7
Acquisition-related costs (Note 11)					$ 190,000,000
Payments of merger related costs, financing activities					17,000,000
Net book value									111,000,000						102,000,000
Repayments of term loan					0		$ 636,000,000	$ 0
Number of series of notes maturing in 2023 | series														2
Investment in government securities (Note 17)				$ 198,000,000	267,000,000		0	0
Proceeds from settlement of government securities (Note 17)				200,000,000	274,000,000		0	0
Other Expense
Debt Instrument [Line Items]
Acquisition-related costs (Note 11)					6,000,000		0	$ 247,000,000
Payments of merger related costs, financing activities					$ 12,000,000
US Government Debt Securities
Debt Instrument [Line Items]
Debt securities														$ 647,000,000
Term Credit Facility
Debt Instrument [Line Items]
Repayments of term loan						$ 500,000,000	636,000,000
Letters of Credit
Debt Instrument [Line Items]
Maximum capacity under credit facility									300,000,000
Letters of credit drawn									93,000,000						75,000,000
Commercial Paper | Commercial Paper Program
Debt Instrument [Line Items]
Credit facility, available amount										$ 1,500,000,000	$ 1,000,000,000
Line of credit, current				$ 800,000,000					$ 1,058,000,000						0
Weighted-average interest rate				5.59%					5.59%
RRIF Loan
Debt Instrument [Line Items]
Interest rate																2.96%
Debt instrument, face amount									$ 14,000,000
Number of locomotives purchased | locomotive																	30
RRIF Loan | Texas Mexican Railway
Debt Instrument [Line Items]
Interest rate																		4.29%
Debt instrument, face amount																		$ 50,000,000
RRIF Loan | Kansas City Southern
Debt Instrument [Line Items]
Debt instrument, face amount																$ 55,000,000
2.450% 10-year Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 1,851,000,000						1,896,000,000
Interest rate				2.45%					2.45%
Debt term				10 years	10 years
3.000% 20-year Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 1,317,000,000						1,348,000,000
Interest rate				3.00%					3.00%
Debt term				20 years	20 years
2.050% 10-year Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 660,000,000						676,000,000
Interest rate				2.05%					2.05%
Debt term				10 years	10 years
3.050% 30-year Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 298,000,000						298,000,000
Interest rate				3.05%					3.05%
Debt term				30 years	30 years
5.41% Senior Secured Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 64,000,000						76,000,000
Interest rate				5.41%					5.41%
Value of locomotive units used as collateral									$ 76,000,000
6.91% Secured Equipment Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 21,000,000						$ 40,000,000
Interest rate				6.91%					6.91%
Value of locomotive units used as collateral									$ 27,000,000
6.99 Finance Lease
Debt Instrument [Line Items]
Interest rate						6.99%									6.99%
Repayments of debt						$ 76,000,000	97,000,000
4.450% 12.5-year Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 0						$ 474,000,000
Interest rate				4.45%					4.45%
Debt term				12 years 6 months	12 years 6 months
4.450% 12.5-year Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount				$ 350,000,000	$ 479,000,000
Interest rate				4.45%					4.45%
Debt term				12 years 6 months	12 years 6 months
3.85% 10-year Senior Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 199,000,000			$ 199,000,000	$ 272,000,000
Interest rate	3.85%			3.85%					3.85%
Debt term				10 years	10 years
3.00% 10-year Senior Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount		$ 439,000,000		$ 439,000,000	$ 592,000,000
Interest rate		3.00%		3.00%					3.00%
Debt term				10 years	10 years
1.589% 2-year Notes
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current									$ 0						$ 1,000,000,000
Interest rate				1.589%					1.589%
Debt term				2 years	2 years
1.589% 2-year Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount					$ 1,000,000,000
Interest rate				1.589%					1.589%
Debt term				2 years	2 years
5.100% 10-year Medium Term Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount							$ 125,000,000
Interest rate						5.10%									5.10%
Debt term						10 years	10 years
4.500% 10-year Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount						$ 250,000,000	$ 313,000,000
Interest rate						4.50%									4.50%
Debt term						10 years	10 years
Senior Notes
Debt Instrument [Line Items]
Debt instrument, face amount			$ 3,014,000,000	$ 104,000,000
Cash consideration paid for debt exchange			1.00
Unamortized premium			30
Senior Notes | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Principal amount exchanged			1,000
Senior Notes | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Principal amount exchanged			$ 970
The Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, available amount				$ 2,200,000,000
5-year Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, available amount												$ 1,100,000,000	$ 1,000,000,000
5-year Credit Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Debt term				5 years	5 years
2-year Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, available amount												$ 1,100,000,000	$ 300,000,000
2-year Credit Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Debt term				2 years	2 years
U.S dollar-denominated
Debt Instrument [Line Items]
Long-term debt and finance lease obligation, Current and non-current				$ 15,764,000,000		$ 12,161,000,000