Condensed Consolidating Financial Information - Interim Condensed Consolidating Balance Sheets (Details) - CAD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 150		$ 338
Accounts receivable, net	759		687
Accounts receivable, inter-company	0		0
Short-term advances to affiliates	0		0
Materials and supplies	156		152
Other current assets	65		97
Total current assets	1,130		1,274
Long-term advances to affiliates	0		0
Investments	201		182
Investments in subsidiaries	0		0
Properties	17,792		17,016
Goodwill and intangible assets	192		187
Pension asset	1,726		1,407
Other assets	68		69
Deferred income taxes	0		0
Total assets	21,109		20,135
Current liabilities
Accounts payable and accrued liabilities	1,189		1,238
Accounts payable, inter-company	0		0
Short-term advances from affiliates	0		0
Long-term debt maturing within one year	480		746
Total current liabilities	1,669		1,984
Pension and other benefit liabilities	746		749
Long-term advances from affiliates	0		0
Other long-term liabilities	232		231
Long-term debt	7,806		7,413
Deferred income taxes	3,528		3,321
Total liabilities	13,981		13,698
Shareholders’ equity
Share capital	2,017		2,032
Additional paid-in capital	47		43
Accumulated other comprehensive (loss) income	(1,654)	$ (1,673)	(1,741)	$ (1,715)	$ (1,740)	$ (1,799)
Retained earnings	6,718		6,103
Total shareholders' equity	7,128		6,437	$ 5,565		$ 4,626
Total liabilities and shareholders’ equity	21,109		20,135
Consolidating Adjustments and Eliminations [Member]
Current assets
Cash and cash equivalents	0		0
Accounts receivable, net	0		0
Accounts receivable, inter-company	(429)		(465)
Short-term advances to affiliates	(5,725)		(6,500)
Materials and supplies	0		0
Other current assets	(28)		0
Total current assets	(6,182)		(6,965)
Long-term advances to affiliates	(1,183)		(1,000)
Investments	0		0
Investments in subsidiaries	(23,094)		(22,745)
Properties	0		0
Goodwill and intangible assets	0		0
Pension asset	0		0
Other assets	0		0
Deferred income taxes	(6)		(3)
Total assets	(30,465)		(30,713)
Current liabilities
Accounts payable and accrued liabilities	(28)		0
Accounts payable, inter-company	(429)		(465)
Short-term advances from affiliates	(5,725)		(6,500)
Long-term debt maturing within one year	0		0
Total current liabilities	(6,182)		(6,965)
Pension and other benefit liabilities	0		0
Long-term advances from affiliates	(1,183)		(1,000)
Other long-term liabilities	0		0
Long-term debt	0		0
Deferred income taxes	(6)		(3)
Total liabilities	(7,371)		(7,968)
Shareholders’ equity
Share capital	(6,665)		(7,767)
Additional paid-in capital	(1,745)		(1,902)
Accumulated other comprehensive (loss) income	1,083		1,325
Retained earnings	(15,767)		(14,401)
Total shareholders' equity	(23,094)		(22,745)
Total liabilities and shareholders’ equity	(30,465)		(30,713)
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Current assets
Cash and cash equivalents	0		0
Accounts receivable, net	0		0
Accounts receivable, inter-company	119		97
Short-term advances to affiliates	0		500
Materials and supplies	0		0
Other current assets	0		0
Total current assets	119		597
Long-term advances to affiliates	1,090		590
Investments	0		0
Investments in subsidiaries	11,366		10,623
Properties	0		0
Goodwill and intangible assets	0		0
Pension asset	0		0
Other assets	0		0
Deferred income taxes	6		3
Total assets	12,581		11,813
Current liabilities
Accounts payable and accrued liabilities	93		82
Accounts payable, inter-company	4		3
Short-term advances from affiliates	5,356		5,291
Long-term debt maturing within one year	0		0
Total current liabilities	5,453		5,376
Pension and other benefit liabilities	0		0
Long-term advances from affiliates	0		0
Other long-term liabilities	0		0
Long-term debt	0		0
Deferred income taxes	0		0
Total liabilities	5,453		5,376
Shareholders’ equity
Share capital	2,017		2,032
Additional paid-in capital	47		43
Accumulated other comprehensive (loss) income	(1,654)		(1,741)
Retained earnings	6,718		6,103
Total shareholders' equity	7,128		6,437
Total liabilities and shareholders’ equity	12,581		11,813
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Current assets
Cash and cash equivalents	103		241
Accounts receivable, net	592		508
Accounts receivable, inter-company	128		153
Short-term advances to affiliates	1,049		1,004
Materials and supplies	123		120
Other current assets	45		31
Total current assets	2,040		2,057
Long-term advances to affiliates	5		0
Investments	35		27
Investments in subsidiaries	11,728		12,122
Properties	9,395		8,982
Goodwill and intangible assets	0		0
Pension asset	1,726		1,407
Other assets	57		56
Deferred income taxes	0		0
Total assets	24,986		24,651
Current liabilities
Accounts payable and accrued liabilities	822		844
Accounts payable, inter-company	297		309
Short-term advances from affiliates	366		1,185
Long-term debt maturing within one year	480		746
Total current liabilities	1,965		3,084
Pension and other benefit liabilities	668		672
Long-term advances from affiliates	1,178		1,000
Other long-term liabilities	114		108
Long-term debt	7,754		7,362
Deferred income taxes	1,941		1,802
Total liabilities	13,620		14,028
Shareholders’ equity
Share capital	538		1,037
Additional paid-in capital	1,653		1,643
Accumulated other comprehensive (loss) income	(1,654)		(1,742)
Retained earnings	10,829		9,685
Total shareholders' equity	11,366		10,623
Total liabilities and shareholders’ equity	24,986		24,651
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Current assets
Cash and cash equivalents	47		97
Accounts receivable, net	167		179
Accounts receivable, inter-company	182		215
Short-term advances to affiliates	4,676		4,996
Materials and supplies	33		32
Other current assets	48		66
Total current assets	5,153		5,585
Long-term advances to affiliates	88		410
Investments	166		155
Investments in subsidiaries	0		0
Properties	8,397		8,034
Goodwill and intangible assets	192		187
Pension asset	0		0
Other assets	11		13
Deferred income taxes	0		0
Total assets	14,007		14,384
Current liabilities
Accounts payable and accrued liabilities	302		312
Accounts payable, inter-company	128		153
Short-term advances from affiliates	3		24
Long-term debt maturing within one year	0		0
Total current liabilities	433		489
Pension and other benefit liabilities	78		77
Long-term advances from affiliates	5		0
Other long-term liabilities	118		123
Long-term debt	52		51
Deferred income taxes	1,593		1,522
Total liabilities	2,279		2,262
Shareholders’ equity
Share capital	6,127		6,730
Additional paid-in capital	92		259
Accumulated other comprehensive (loss) income	571		417
Retained earnings	4,938		4,716
Total shareholders' equity	11,728		12,122
Total liabilities and shareholders’ equity	$ 14,007		$ 14,384