Condensed Consolidating Financial Information - Condensed Consolidating Statements of Cash Flows (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	CAD 2,182	CAD 2,089	CAD 2,459
Investing activities
Additions to properties	(1,340)	(1,182)	(1,522)
Proceeds from the sale of property, plant and equipment	0	0	281
Proceeds from sale of properties and other assets	42	116	114
Advances to affiliates	0	0	0
Repayment of advances to affiliates	0	0	0
Capital contributions to affiliates	0	0	0
Repurchase of share capital from affiliates	0	0	0
Other	3	(3)	4
Cash used in investing activities	(1,295)	(1,069)	(1,123)
Financing activities
Dividends paid	(310)	(255)	(226)
Issuance of share capital	0	0	0
Return of share capital to affiliates	0	0	0
Issuance of CP Common Shares	45	21	43
Purchase of CP Common shares	(381)	(1,210)	(2,787)
Issuance of long-term debt, excluding commercial paper	0	0	3,411
Repayment of long-term debt, excluding commercial paper	(32)	(38)	(505)
Net repayment of commercial paper	0	(8)	(893)
Advances from affiliates	0	0	0
Repayment of advances from affiliates	0	0	0
Settlement of forward starting swaps	(22)	0	0
Other	0	(3)	0
Cash used in financing activities	(700)	(1,493)	(957)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	(13)	(13)	45
Cash position
Increase (decrease) in cash and cash equivalents	174	(486)	424
Cash and cash equivalents at beginning of year	164	650	226
Cash and cash equivalents at end of year	338	164	650
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	(479)	(469)	(2,548)
Investing activities
Additions to properties	0	0	0
Proceeds from the sale of property, plant and equipment			0
Proceeds from sale of properties and other assets	0	0	0
Advances to affiliates	2,668	1,203	3,264
Repayment of advances to affiliates	(485)	(222)	(1,804)
Capital contributions to affiliates	1,039	472	1,655
Repurchase of share capital from affiliates	(156)	(8)	(1,210)
Other	0	0	0
Cash used in investing activities	3,066	1,445	1,905
Financing activities
Dividends paid	479	469	2,548
Issuance of share capital	(1,039)	(472)	(1,655)
Return of share capital to affiliates	156	8	1,210
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	0
Advances from affiliates	(2,668)	(1,203)	(3,264)
Repayment of advances from affiliates	485	222	1,804
Settlement of forward starting swaps	0
Other		0
Cash used in financing activities	(2,587)	(976)	643
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	0	0	0
Cash position
Increase (decrease) 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	338	255	2,283
Investing activities
Additions to properties	0	0	0
Proceeds from the sale of property, plant and equipment			0
Proceeds from sale of properties and other assets	0	0	0
Advances to affiliates	(590)	0	(1,133)
Repayment of advances to affiliates	0	0	0
Capital contributions to affiliates	0	0	0
Repurchase of share capital from affiliates	0	0	0
Other	0	0	0
Cash used in investing activities	(590)	0	(1,133)
Financing activities
Dividends paid	(310)	(255)	(226)
Issuance of share capital	0	0	0
Return of share capital to affiliates	0	0	0
Issuance of CP Common Shares	45	21	43
Purchase of CP Common shares	(381)	(1,210)	(2,787)
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	0
Advances from affiliates	1,383	1,189	1,820
Repayment of advances from affiliates	(485)	0	0
Settlement of forward starting swaps	0
Other		0
Cash used in financing activities	252	(255)	(1,150)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	0	0	0
Cash position
Increase (decrease) 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	1,334	1,424	1,650
Investing activities
Additions to properties	(950)	(728)	(766)
Proceeds from the sale of property, plant and equipment			0
Proceeds from sale of properties and other assets	29	102	103
Advances to affiliates	(550)	(664)	(311)
Repayment of advances to affiliates	242	222	804
Capital contributions to affiliates	(1,039)	(472)	(1,655)
Repurchase of share capital from affiliates	156	8	1,210
Other	5	0	6
Cash used in investing activities	(2,107)	(1,532)	(609)
Financing activities
Dividends paid	(310)	(255)	(2,272)
Issuance of share capital	0	0	0
Return of share capital to affiliates	0	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			3,411
Repayment of long-term debt, excluding commercial paper	(32)	(24)	(461)
Net repayment of commercial paper		(8)	(893)
Advances from affiliates	1,285	0	500
Repayment of advances from affiliates	0	0	(1,000)
Settlement of forward starting swaps	(22)
Other		(3)
Cash used in financing activities	921	(290)	(715)
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	(7)	(4)	24
Cash position
Increase (decrease) in cash and cash equivalents	141	(402)	350
Cash and cash equivalents at beginning of year	100	502	152
Cash and cash equivalents at end of year	241	100	502
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Cash provided by operating activities	989	879	1,074
Investing activities
Additions to properties	(390)	(454)	(756)
Proceeds from the sale of property, plant and equipment			281
Proceeds from sale of properties and other assets	13	14	11
Advances to affiliates	(1,528)	(539)	(1,820)
Repayment of advances to affiliates	243	0	1,000
Capital contributions to affiliates	0	0	0
Repurchase of share capital from affiliates	0	0	0
Other	(2)	(3)	(2)
Cash used in investing activities	(1,664)	(982)	(1,286)
Financing activities
Dividends paid	(169)	(214)	(276)
Issuance of share capital	1,039	472	1,655
Return of share capital to affiliates	(156)	(8)	(1,210)
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	(14)	(44)
Net repayment of commercial paper		0	0
Advances from affiliates	0	14	944
Repayment of advances from affiliates	0	(222)	(804)
Settlement of forward starting swaps	0
Other		0
Cash used in financing activities	714	28	265
Effect of foreign currency fluctuations on U.S. dollar-denominated cash and cash equivalents	(6)	(9)	21
Cash position
Increase (decrease) in cash and cash equivalents	33	(84)	74
Cash and cash equivalents at beginning of year	64	148	74
Cash and cash equivalents at end of year	CAD 97	CAD 64	CAD 148