Business acquisitions - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions, $ in Millions													1 Months Ended	3 Months Ended	5 Months Ended	7 Months Ended	12 Months Ended
	Dec. 14, 2021 CAD ($) shares	Dec. 14, 2021 USD ($) $ / shares shares	Dec. 02, 2021 CAD ($)	Dec. 02, 2021 USD ($)	Nov. 24, 2021 CAD ($)	Sep. 15, 2021 CAD ($)	Sep. 15, 2021 USD ($)	May 21, 2021 CAD ($)	May 21, 2021 USD ($)	Dec. 22, 2020 CAD ($) mi	Jun. 03, 2020 CAD ($)	Dec. 30, 2019 CAD ($) mi	Dec. 31, 2021 CAD ($)	Dec. 31, 2021 CAD ($)	Jun. 02, 2020 CAD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2021 CAD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2019 CAD ($)	Dec. 14, 2021 USD ($)	Aug. 10, 2021 USD ($)	Mar. 21, 2021 mi
Business Acquisition [Line Items]
Merger termination fee																	$ 845	$ 0	$ 0
Amount remitted																	12,299	0	0
Proceeds from issuance of long-term debt			$ 8,600	$ 6,700	$ 2,200									$ 10,700
Investment													$ 42,309	42,309		$ 0	42,309	0
Equity loss of Kansas City Southern																	141	0	0
Acquisition-related costs																	247	0	0
Financing fees																	51	0	0
Cash consideration																	0	398	0
Goodwill													328	328		329	328	329	$ 194
DRTP
Business Acquisition [Line Items]
Ownership percentage										100.00%
Kansas City Southern
Business Acquisition [Line Items]
Amount remitted						$ 1,773	$ 1,400
Investment													42,309	42,309			42,309
Equity loss of Kansas City Southern																	141
Amortization of basis difference																	8
Basis difference													30,000	$ 30,000			30,000
Kansas City Southern
Business Acquisition [Line Items]
Number of route miles acquired | mi																						7,100
Merger termination fee							$ 700	$ 845	$ 700
Enterprise value																					$ 31,000
Assumption of debt																					$ 3,800
Exchange ratio																					2.884	2.445
Stock issued (in shares) | shares	262.6	262.6
Stock issued, value	$ 23,500																			$ 18,300
Cash consideration	10,500	$ 8,200
Total consideration	$ 36,000	$ 28,000
Acquisition-related costs																	599
Financing fees																	51
Percentage ownership acquired	100.00%																			100.00%
Kansas City Southern | Common stock
Business Acquisition [Line Items]
Consideration transferred, per share (in CAD or USD per share) | $ / shares		$ 90
Kansas City Southern | Preferred stock
Business Acquisition [Line Items]
Consideration transferred, per share (in CAD or USD per share) | $ / shares		$ 37.50
Kansas City Southern | Purchased services and other
Business Acquisition [Line Items]
Acquisition-related costs																	183
Kansas City Southern | Other expense (income)
Business Acquisition [Line Items]
Acquisition-related costs																	$ 247
Kansas City Southern | Equity loss of Kansas City Southern
Business Acquisition [Line Items]
Acquisition-related costs													$ 169
DRTP
Business Acquisition [Line Items]
Investment										$ 13
Percentage ownership acquired										83.50%
Cash consideration										$ 398
Reduction of operating costs related to movements through the DRTP																		$ (34)
Miles of rail tunnel acquired | mi										1.6
Previously held equity interest, Percentage										16.50%
Gain on remeasurement of DRTP equity interest										$ 68
Fair value of previously held equity method investment										81
Goodwill										90
Intangible assets										4
Acquired cash and equivalents										$ 6
CMQ
Business Acquisition [Line Items]
Percentage ownership acquired												100.00%
Cash consideration												$ 174
Additional consideration paid, working capital adjustment															$ 3
CMQ U.S.
Business Acquisition [Line Items]
Fair value of previously held equity method investment											$ 138
Goodwill											52
Intangible assets											27
Acquired cash and equivalents											$ 22
Additional consideration paid, working capital adjustment															$ 3
Measurement period adjustment, Other long-term liabilities																$ 1
Amortization period of intangible assets with finite lives (years)											20 years
Canada | CMQ
Business Acquisition [Line Items]
Miles of rail lines acquired | mi												237
United States | CMQ
Business Acquisition [Line Items]
Miles of rail lines acquired | mi												244