Subsequent events - Narrative (Details) $ in Millions			3 Months Ended
	Apr. 19, 2023 USD ($) series	Apr. 13, 2023 CAD ($)	Mar. 31, 2023 CAD ($)	Mar. 31, 2022 CAD ($)	Apr. 24, 2023 USD ($) series	Dec. 31, 2022 CAD ($)
Subsequent Event [Line Items]
Investment in Kansas City Southern			$ 44,955			$ 45,091
Deferred income tax expense (recovery)			(24)	$ 1
Income (loss) from equity method investments
Subsequent Event [Line Items]
Earnings from equity method investment			200	200
Subsequent Event
Subsequent Event [Line Items]
Investment in Kansas City Southern		$ 44,400
Number of series of notes included in debt exchange | series	7
Series of notes maturing in 2023 | series					2
Subsequent Event | US Government Debt Securities
Subsequent Event [Line Items]
Non-callable government securities					$ 647,000,000
Subsequent Event | Senior Notes
Subsequent Event [Line Items]
Cash consideration paid for debt exchange	$ 1.00
Premium on debt exchange	30
Principal and interest outstanding	3,014,000,000
Subsequent Event | Senior Notes | KCS Notes Due In May 2023
Subsequent Event [Line Items]
Principal and interest outstanding					445,000,000
Subsequent Event | Senior Notes | KCS Notes Due In November 2023
Subsequent Event [Line Items]
Principal and interest outstanding					$ 203,000,000
Subsequent Event | Senior Notes | Early participation date
Subsequent Event [Line Items]
Principal amount exchanged	1,000
Subsequent Event | Senior Notes | After the early participation date, before the expiration date
Subsequent Event [Line Items]
Principal amount exchanged	$ 970
Kansas City Southern Debt Exchange
Subsequent Event [Line Items]
Acquisition-related costs			3
KCS
Subsequent Event [Line Items]
Net remeasurement loss on equity method investment				7,200
Deferred income tax expense (recovery)				7,800
Acquisition-related costs			$ 15	$ 20
KCS | Subsequent Event
Subsequent Event [Line Items]
Fair value of previously held equity method investment		37,200
Net remeasurement loss on equity method investment		7,200
Deferred income tax expense (recovery)		7,800
Cash and cash equivalents		300
Intangible assets		2,600
Goodwill		18,500
KCS | Subsequent Event | Contracts and Customer Relationships
Subsequent Event [Line Items]
Intangible assets		$ 2,600
KCS | Subsequent Event | Contracts and Customer Relationships | Minimum
Subsequent Event [Line Items]
Amortization periods of intangible assets acquired		22 years
KCS | Subsequent Event | Contracts and Customer Relationships | Maximum
Subsequent Event [Line Items]
Amortization periods of intangible assets acquired		9 years
KCS | Subsequent Event | Concession Rights
Subsequent Event [Line Items]
Renewal term of concession rights and related assets		50 years
Intangible assets		$ 8,300