Condensed Consolidating Financial Information - Condensed Consolidating Statements of Cash Flows (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	$ 2,712	$ 2,182	$ 2,089
Investing activities
Additions to properties	(1,551)	(1,340)	(1,182)
Proceeds from sale of properties and other assets	78	42	116
Advances to affiliates	0	0	0
Repayment of advances to affiliates	0	0	0
Capital contributions to affiliates		0	0
Repurchase of share capital from affiliates	0	0	0
Other	15	3	(3)
Cash used in investing activities	(1,458)	(1,295)	(1,069)
Financing activities
Dividends paid	(348)	(310)	(255)
Issuance of share capital		0	0
Return of share capital to affiliates	0	0	0
Issuance of CP Common Shares	24	45	21
Purchase of CP Common shares	(1,103)	(381)	(1,210)
Issuance of long-term debt, excluding commercial paper	638
Repayment of long-term debt, excluding commercial paper	(753)	(32)	(38)
Net repayment of commercial paper	0	0	(8)
Advances from affiliates	0	0	0
Repayment of advances from affiliates	0	0	0
Settlement of forward starting swaps on de-designation	0	(22)	0
Other	0	0	(3)
Cash used in financing activities	(1,542)	(700)	(1,493)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	11	(13)	(13)
Cash position
(Decrease) increase in cash and cash equivalents	(277)	174	(486)
Cash and cash equivalents at beginning of year	338	164	650
Cash and cash equivalents at end of year	61	338	164
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	(700)	(479)	(469)
Investing activities
Additions to properties	0	0	0
Proceeds from sale of properties and other assets	0	0	0
Advances to affiliates	820	2,668	1,203
Repayment of advances to affiliates	(866)	(485)	(222)
Capital contributions to affiliates		1,039	472
Repurchase of share capital from affiliates	(1,464)	(156)	(8)
Other	0	0	0
Cash used in investing activities	(1,510)	3,066	1,445
Financing activities
Dividends paid	700	479	469
Issuance of share capital		(1,039)	(472)
Return of share capital to affiliates	1,464	156	8
Issuance of CP Common Shares	0	0	0
Purchase of CP Common shares	0	0	0
Issuance of long-term debt, excluding commercial paper	0
Repayment of long-term debt, excluding commercial paper	0	0	0
Net repayment of commercial paper			0
Advances from affiliates	(820)	(2,668)	(1,203)
Repayment of advances from affiliates	866	485	222
Settlement of forward starting swaps on de-designation		0
Other			0
Cash used in financing activities	2,210	(2,587)	(976)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	0	0	0
Cash position
(Decrease) increase in cash and cash equivalents	0	0	0
Cash and cash equivalents at beginning of year	0	0	0
Cash and cash equivalents at end of year	0	0	0
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	316	338	255
Investing activities
Additions to properties	0	0	0
Proceeds from sale of properties and other assets	0	0	0
Advances to affiliates	0	(590)	0
Repayment of advances to affiliates	0	0	0
Capital contributions to affiliates		0	0
Repurchase of share capital from affiliates	500	0	0
Other	0	0	0
Cash used in investing activities	500	(590)	0
Financing activities
Dividends paid	(348)	(310)	(255)
Issuance of share capital		0	0
Return of share capital to affiliates	0	0	0
Issuance of CP Common Shares	24	45	21
Purchase of CP Common shares	(1,103)	(381)	(1,210)
Issuance of long-term debt, excluding commercial paper	0
Repayment of long-term debt, excluding commercial paper	0	0	0
Net repayment of commercial paper			0
Advances from affiliates	820	1,383	1,189
Repayment of advances from affiliates	(209)	(485)	0
Settlement of forward starting swaps on de-designation		0
Other			0
Cash used in financing activities	(816)	252	(255)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	0	0	0
Cash position
(Decrease) increase in cash and cash equivalents	0	0	0
Cash and cash equivalents at beginning of year	0	0	0
Cash and cash equivalents at end of year	0	0	0
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	1,968	1,334	1,424
Investing activities
Additions to properties	(971)	(950)	(728)
Proceeds from sale of properties and other assets	35	29	102
Advances to affiliates	(611)	(550)	(664)
Repayment of advances to affiliates	0	242	222
Capital contributions to affiliates		(1,039)	(472)
Repurchase of share capital from affiliates	964	156	8
Other	18	5	0
Cash used in investing activities	(565)	(2,107)	(1,532)
Financing activities
Dividends paid	(348)	(310)	(255)
Issuance of share capital		0	0
Return of share capital to affiliates	(500)	0	0
Issuance of CP Common Shares	0	0	0
Purchase of CP Common shares	0	0	0
Issuance of long-term debt, excluding commercial paper	638
Repayment of long-term debt, excluding commercial paper	(753)	(32)	(24)
Net repayment of commercial paper			(8)
Advances from affiliates	0	1,285	0
Repayment of advances from affiliates	(657)	0	0
Settlement of forward starting swaps on de-designation		(22)
Other			(3)
Cash used in financing activities	(1,620)	921	(290)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	18	(7)	(4)
Cash position
(Decrease) increase in cash and cash equivalents	(199)	141	(402)
Cash and cash equivalents at beginning of year	241	100	502
Cash and cash equivalents at end of year	42	241	100
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	1,128	989	879
Investing activities
Additions to properties	(580)	(390)	(454)
Proceeds from sale of properties and other assets	43	13	14
Advances to affiliates	(209)	(1,528)	(539)
Repayment of advances to affiliates	866	243	0
Capital contributions to affiliates		0	0
Repurchase of share capital from affiliates	0	0	0
Other	(3)	(2)	(3)
Cash used in investing activities	117	(1,664)	(982)
Financing activities
Dividends paid	(352)	(169)	(214)
Issuance of share capital		1,039	472
Return of share capital to affiliates	(964)	(156)	(8)
Issuance of CP Common Shares	0	0	0
Purchase of CP Common shares	0	0	0
Issuance of long-term debt, excluding commercial paper	0
Repayment of long-term debt, excluding commercial paper	0	0	(14)
Net repayment of commercial paper			0
Advances from affiliates	0	0	14
Repayment of advances from affiliates	0	0	(222)
Settlement of forward starting swaps on de-designation		0
Other			0
Cash used in financing activities	(1,316)	714	28
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	(7)	(6)	(9)
Cash position
(Decrease) increase in cash and cash equivalents	(78)	33	(84)
Cash and cash equivalents at beginning of year	97	64	148
Cash and cash equivalents at end of year	$ 19	$ 97	$ 64