Condensed Consolidating Financial Information - Interim Condensed Consolidating Statements of Cash Flows (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	$ 711	$ 611	$ 1,108	$ 922
Investing activities
Additions to properties	(413)	(346)	(654)	(576)
Proceeds from sale of properties and other assets	5	13	9	16
Advances to affiliates	0	0	0	0
Repayment of advances to affiliates		0	0
Capital contributions to affiliates		0		0
Repurchase of share capital from affiliates	0		0
Other investing activities	0	0	(1)	5
Cash used in investing activities	(408)	(333)	(646)	(555)
Financing activities
Dividends paid	(81)	(73)	(163)	(146)
Issuance of share capital		0		0
Return of share capital to affiliates	0		0
Issuance of CP Common Shares	4	9	12	37
Purchase of CP Common Shares	(261)	(142)	(559)	(142)
Issuance of long-term debt, excluding commercial paper	638		638
Repayment of long-term debt, excluding commercial paper	(734)	(9)	(739)	(14)
Proceeds from issuance of commercial paper	53	0	53	0
Advances from affiliates	0	0	0	0
Repayments of advances from affiliates		0	0
Settlement of forward starting swaps on de-designation	0	(22)	0	(22)
Cash (used in) provided by financing activities	(381)	(237)	(758)	(287)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	4	(4)	9	(6)
Cash position
(Decrease) increase in cash and cash equivalents	(74)	37	(287)	74
Cash and cash equivalents at beginning of period	125	201	338	164
Cash and cash equivalents at end of period	51	238	51	238
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	(201)	(191)	(396)	(292)
Investing activities
Additions to properties	0	0	0	0
Proceeds from sale of properties and other assets	0	0	0	0
Advances to affiliates	262	2,612	562	2,896
Repayment of advances to affiliates		(2)	(495)
Capital contributions to affiliates		945		1,013
Repurchase of share capital from affiliates	(124)		(547)
Other investing activities		0	0	0
Cash used in investing activities	138	3,555	(480)	3,909
Financing activities
Dividends paid	201	191	396	292
Issuance of share capital		(945)		(1,013)
Return of share capital to affiliates	124		547
Issuance of CP Common Shares	0	0	0	0
Purchase of CP Common Shares	0	0	0	0
Issuance of long-term debt, excluding commercial paper	0		0
Repayment of long-term debt, excluding commercial paper	0	0	0	0
Proceeds from issuance of commercial paper	0		0
Advances from affiliates	(262)	(2,612)	(562)	(2,896)
Repayments of advances from affiliates		2	495
Settlement of forward starting swaps on de-designation		0		0
Cash (used in) provided by financing activities	63	(3,364)	876	(3,617)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	0	0	0	0
Cash position
(Decrease) increase in cash and cash equivalents	0	0	0	0
Cash and cash equivalents at beginning of period	0	0	0	0
Cash and cash equivalents at end of period	0	0	0	0
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	83	95	148	158
Investing activities
Additions to properties	0	0	0	0
Proceeds from sale of properties and other assets	0	0	0	0
Advances to affiliates	0	(1,086)	0	(1,238)
Repayment of advances to affiliates		0	0
Capital contributions to affiliates		0		0
Repurchase of share capital from affiliates	0		0
Other investing activities		0	0	0
Cash used in investing activities	0	(1,086)	0	(1,238)
Financing activities
Dividends paid	(81)	(73)	(163)	(146)
Issuance of share capital		0		0
Return of share capital to affiliates	0		0
Issuance of CP Common Shares	4	9	12	37
Purchase of CP Common Shares	(261)	(142)	(559)	(142)
Issuance of long-term debt, excluding commercial paper	0		0
Repayment of long-term debt, excluding commercial paper	0	0	0	0
Proceeds from issuance of commercial paper	0		0
Advances from affiliates	255	1,197	562	1,331
Repayments of advances from affiliates		0	0
Settlement of forward starting swaps on de-designation		0		0
Cash (used in) provided by financing activities	(83)	991	(148)	1,080
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	0	0	0	0
Cash position
(Decrease) increase in cash and cash equivalents	0	0	0	0
Cash and cash equivalents at beginning of period	0	0	0	0
Cash and cash equivalents at end of period	0	0	0	0
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	501	468	893	553
Investing activities
Additions to properties	(276)	(192)	(398)	(301)
Proceeds from sale of properties and other assets	3	5	6	6
Advances to affiliates	(255)	(553)	(562)	(551)
Repayment of advances to affiliates		2	0
Capital contributions to affiliates		(945)		(1,013)
Repurchase of share capital from affiliates	124		547
Other investing activities		1	0	6
Cash used in investing activities	(404)	(1,682)	(407)	(1,853)
Financing activities
Dividends paid	(81)	(73)	(163)	(146)
Issuance of share capital		0		0
Return of share capital to affiliates	0		0
Issuance of CP Common Shares	0	0	0	0
Purchase of CP Common Shares	0	0	0	0
Issuance of long-term debt, excluding commercial paper	638		638
Repayment of long-term debt, excluding commercial paper	(734)	(9)	(739)	(14)
Proceeds from issuance of commercial paper	53		53
Advances from affiliates	7	1,415	0	1,564
Repayments of advances from affiliates		0	(495)
Settlement of forward starting swaps on de-designation		(22)		(22)
Cash (used in) provided by financing activities	(117)	1,311	(706)	1,382
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	(3)	(2)	(1)	(4)
Cash position
(Decrease) increase in cash and cash equivalents	(23)	95	(221)	78
Cash and cash equivalents at beginning of period	43	83	241	100
Cash and cash equivalents at end of period	20	178	20	178
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	328	239	463	503
Investing activities
Additions to properties	(137)	(154)	(256)	(275)
Proceeds from sale of properties and other assets	2	8	3	10
Advances to affiliates	(7)	(973)	0	(1,107)
Repayment of advances to affiliates		0	495
Capital contributions to affiliates		0		0
Repurchase of share capital from affiliates	0		0
Other investing activities		(1)	(1)	(1)
Cash used in investing activities	(142)	(1,120)	241	(1,373)
Financing activities
Dividends paid	(120)	(118)	(233)	(146)
Issuance of share capital		945		1,013
Return of share capital to affiliates	(124)		(547)
Issuance of CP Common Shares	0	0	0	0
Purchase of CP Common Shares	0	0	0	0
Issuance of long-term debt, excluding commercial paper	0		0
Repayment of long-term debt, excluding commercial paper	0	0	0	0
Proceeds from issuance of commercial paper	0		0
Advances from affiliates	0	0	0	1
Repayments of advances from affiliates		(2)	0
Settlement of forward starting swaps on de-designation		0		0
Cash (used in) provided by financing activities	(244)	825	(780)	868
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	7	(2)	10	(2)
Cash position
(Decrease) increase in cash and cash equivalents	(51)	(58)	(66)	(4)
Cash and cash equivalents at beginning of period	82	118	97	64
Cash and cash equivalents at end of period	$ 31	$ 60	$ 31	$ 60