Business Acquisition (Details) $ / shares in Units, $ in Millions, $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2021 CAD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 CAD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2020 CAD ($)	Sep. 30, 2021 CAD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 CAD ($)	Sep. 15, 2021 USD ($) $ / shares shares	Aug. 10, 2021 USD ($) shares	Mar. 21, 2021 USD ($) shares
Business Acquisition [Line Items]
Merger termination fee	$ 0				$ 0	$ 845	$ 700	$ 0
Payment to Kansas City Southern	1,773				0	1,773		0
Acquisition-related costs	83				0	295		0
Acquisition-related costs, Financing fees	0				$ 0	$ 45		$ 0
KCS
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity						Kansas City Southern	Kansas City Southern
KCS enterprise value										$ 31,000	$ 29,000
Assumption of KCS debt										$ 3,800	$ 3,800
Merger termination fee			$ 845	$ 700
Exchange ratio of CP common share to KCS common share | shares									2.884	2.884	2.445
KCS termination fee with CN									$ 700
CP Merger Termination Fee Refund									700
Payment to Kansas City Southern	1,773	$ 1,400
Acquisition-related costs	98					$ 442
Acquisition-related costs	83					295
Acquisition-related costs, Financing fees	0					45
Merger Agreement termination fee									700
Merger Agreement termination fee, Required regulatory approval									$ 1,000
KCS | Purchased services and other
Business Acquisition [Line Items]
Acquisition-related costs	$ 15					$ 147
KCS | Common shares
Business Acquisition [Line Items]
Cash for KCS share held | $ / shares									$ 90
KCS | Preferred Stock
Business Acquisition [Line Items]
Cash for KCS share held | $ / shares									$ 37.50