Debt - Narrative (Details) $ in Millions					12 Months Ended
	Jun. 24, 2024 USD ($)	Nov. 15, 2023 USD ($)	May 15, 2023 USD ($)	Apr. 19, 2023 USD ($) series	Dec. 31, 2024 CAD ($) institution	Dec. 31, 2024 USD ($) institution	Dec. 31, 2023 CAD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jul. 12, 2023 USD ($)	Apr. 24, 2023 USD ($) series	Feb. 21, 2012 USD ($)	Dec. 31, 2011 locomotive	Jun. 28, 2005 USD ($)
Debt Instrument [Line Items]
Long-term debt, gross										$ 14,598,000,000	$ 15,764,000,000
Debt, long-term, short-term and finance lease obligation					$ 22,713		$ 22,598
Annual maturities and principal repayments in 2024					2,805
Annual maturities and principal repayments in 2025					2,165
Annual maturities and principal repayments in 2026					7
Annual maturities and principal repayments in 2027					1,927
Annual maturities and principal repayments in 2028					1,020
Annual maturities and principal repayments thereafter					15,354
Extinguishment of debt, principal amount					241	$ 176,000,000
Gain (loss) on extinguishment of debt					22
Number of series of notes | series				7
Payments of merger related costs, financing activities							17
Number of locomotives purchased | locomotive															30
Net book value					$ 102		111
Number of financial institutions | institution					15	15
Number of series of notes maturing in 2023 | series													2
Investment in government securities					$ 0		267	$ 198,000,000	$ 0
Proceeds from settlement of government securities					0		274	$ 200,000,000	$ 0
Other Expense
Debt Instrument [Line Items]
Payments of merger related costs, financing activities							12
US Government Debt Securities
Debt Instrument [Line Items]
Debt securities													$ 647,000,000
Letters of Credit
Debt Instrument [Line Items]
Letters of credit drawn					95		93
Maximum capacity under credit facility					300
Commercial Paper | Commercial Paper Program
Debt Instrument [Line Items]
Credit facility, available amount												$ 1,500,000,000
Line of credit, current					$ 1,586		$ 1,058			$ 1,102,000,000	$ 800,000,000
Weighted-average interest rate					4.75%		5.59%			4.75%	5.59%
RRIF Loan
Debt Instrument [Line Items]
Interest rate														2.96%
Debt instrument, face amount					$ 14
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
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Interest rate					5.57%					5.57%
5.41% Senior Secured Notes
Debt Instrument [Line Items]
Long-term debt, gross					$ 0		$ 64
Interest rate					5.41%					5.41%
5.41% Senior Secured Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount					$ 66	$ 48,000,000
Interest rate					5.41%					5.41%
6.91% Secured Equipment Notes
Debt Instrument [Line Items]
Long-term debt, gross					$ 0		21
Interest rate					6.91%					6.91%
6.91% Secured Equipment Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount						21,000,000
Interest rate					6.91%					6.91%
4.450% 12.5-year Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount					$ 479	$ 350,000,000
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			$ 272	$ 199,000,000
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		$ 592	$ 439,000,000
Interest rate			3.00%		3.00%					3.00%
Debt term					10 years	10 years
1.589% 2-year Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount					$ 1,000
Interest rate					1.589%					1.589%
Debt term					2 years	2 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									1,100,000,000
5-year Credit Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Debt term	5 years				5 years	5 years
2-year Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, available amount	$ 1,100,000,000									1,100,000,000
2-year Credit Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Debt term	2 years				2 years	2 years
Long-term debt					$ 288					$ 200,000,000
1.350% 3-year Notes
Debt Instrument [Line Items]
Long-term debt, gross					$ 0		$ 1,983
Interest rate					1.35%					1.35%
Debt term					3 years	3 years
1.350% 3-year Notes | Senior Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount					$ 2,002	$ 1,429,000,000
Interest rate					1.35%					1.35%
Debt term					3 years	3 years