Condensed Consolidating Financial Information - Interim Condensed Consolidating Balance Sheets (Details) - CAD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 51		$ 338
Accounts receivable, net	686		687
Accounts receivable, inter-company	0		0
Short-term advances to affiliates	0		0
Materials and supplies	160		152
Other current assets	103		97
Total current assets	1,000		1,274
Long-term advances to affiliates	0		0
Investments	193		182
Investments in subsidiaries	0		0
Properties	17,616		17,016
Goodwill and intangible assets	196		187
Pension asset	1,616		1,407
Other assets	64		69
Deferred income taxes	0		0
Total assets	20,685		20,135
Current liabilities
Accounts payable and accrued liabilities	1,213		1,238
Accounts payable, inter-company	0		0
Short-term advances from affiliates	0		0
Long-term debt maturing within one year	547		746
Total current liabilities	1,760		1,984
Pension and other benefit liabilities	749		749
Long-term advances from affiliates	0		0
Other long-term liabilities	218		231
Long-term debt	7,936		7,413
Deferred income taxes	3,448		3,321
Total liabilities	14,111		13,698
Shareholders’ equity
Share capital	2,013		2,032
Additional paid-in capital	45		43
Accumulated other comprehensive (loss) income	(1,673)	$ (1,705)	(1,741)	$ (1,740)	$ (1,769)	$ (1,799)
Retained earnings	6,189		6,103
Total shareholders' equity	6,574		6,437	$ 5,333		$ 4,626
Total liabilities and shareholders’ equity	20,685		20,135
Consolidating Adjustments and Eliminations [Member]
Current assets
Cash and cash equivalents	0		0
Accounts receivable, net	0		0
Accounts receivable, inter-company	(453)		(465)
Short-term advances to affiliates	(6,363)		(6,500)
Materials and supplies	0		0
Other current assets	(56)		0
Total current assets	(6,872)		(6,965)
Long-term advances to affiliates	(1,185)		(1,000)
Investments	0		0
Investments in subsidiaries	(23,225)		(22,745)
Properties	0		0
Goodwill and intangible assets	0		0
Pension asset	0		0
Other assets	0		0
Deferred income taxes	(5)		(3)
Total assets	(31,287)		(30,713)
Current liabilities
Accounts payable and accrued liabilities	(56)		0
Accounts payable, inter-company	(453)		(465)
Short-term advances from affiliates	(6,363)		(6,500)
Long-term debt maturing within one year	0		0
Total current liabilities	(6,872)		(6,965)
Pension and other benefit liabilities	0		0
Long-term advances from affiliates	(1,185)		(1,000)
Other long-term liabilities	0		0
Long-term debt	0		0
Deferred income taxes	(5)		(3)
Total liabilities	(8,062)		(7,968)
Shareholders’ equity
Share capital	(7,345)		(7,767)
Additional paid-in capital	(1,798)		(1,902)
Accumulated other comprehensive (loss) income	1,019		1,325
Retained earnings	(15,101)		(14,401)
Total shareholders' equity	(23,225)		(22,745)
Total liabilities and shareholders’ equity	(31,287)		(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	114		97
Short-term advances to affiliates	0		500
Materials and supplies	0		0
Other current assets	0		0
Total current assets	114		597
Long-term advances to affiliates	1,090		590
Investments	0		0
Investments in subsidiaries	11,320		10,623
Properties	0		0
Goodwill and intangible assets	0		0
Pension asset	0		0
Other assets	0		0
Deferred income taxes	5		3
Total assets	12,529		11,813
Current liabilities
Accounts payable and accrued liabilities	93		82
Accounts payable, inter-company	4		3
Short-term advances from affiliates	5,858		5,291
Long-term debt maturing within one year	0		0
Total current liabilities	5,955		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	5,955		5,376
Shareholders’ equity
Share capital	2,013		2,032
Additional paid-in capital	45		43
Accumulated other comprehensive (loss) income	(1,673)		(1,741)
Retained earnings	6,189		6,103
Total shareholders' equity	6,574		6,437
Total liabilities and shareholders’ equity	12,529		11,813
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Current assets
Cash and cash equivalents	20		241
Accounts receivable, net	510		508
Accounts receivable, inter-company	134		153
Short-term advances to affiliates	1,550		1,004
Materials and supplies	126		120
Other current assets	78		31
Total current assets	2,418		2,057
Long-term advances to affiliates	5		0
Investments	25		27
Investments in subsidiaries	11,905		12,122
Properties	9,220		8,982
Goodwill and intangible assets	0		0
Pension asset	1,616		1,407
Other assets	55		56
Deferred income taxes	0		0
Total assets	25,244		24,651
Current liabilities
Accounts payable and accrued liabilities	871		844
Accounts payable, inter-company	315		309
Short-term advances from affiliates	502		1,185
Long-term debt maturing within one year	547		746
Total current liabilities	2,235		3,084
Pension and other benefit liabilities	669		672
Long-term advances from affiliates	1,180		1,000
Other long-term liabilities	101		108
Long-term debt	7,882		7,362
Deferred income taxes	1,857		1,802
Total liabilities	13,924		14,028
Shareholders’ equity
Share capital	1,037		1,037
Additional paid-in capital	1,650		1,643
Accumulated other comprehensive (loss) income	(1,674)		(1,742)
Retained earnings	10,307		9,685
Total shareholders' equity	11,320		10,623
Total liabilities and shareholders’ equity	25,244		24,651
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Current assets
Cash and cash equivalents	31		97
Accounts receivable, net	176		179
Accounts receivable, inter-company	205		215
Short-term advances to affiliates	4,813		4,996
Materials and supplies	34		32
Other current assets	81		66
Total current assets	5,340		5,585
Long-term advances to affiliates	90		410
Investments	168		155
Investments in subsidiaries	0		0
Properties	8,396		8,034
Goodwill and intangible assets	196		187
Pension asset	0		0
Other assets	9		13
Deferred income taxes	0		0
Total assets	14,199		14,384
Current liabilities
Accounts payable and accrued liabilities	305		312
Accounts payable, inter-company	134		153
Short-term advances from affiliates	3		24
Long-term debt maturing within one year	0		0
Total current liabilities	442		489
Pension and other benefit liabilities	80		77
Long-term advances from affiliates	5		0
Other long-term liabilities	117		123
Long-term debt	54		51
Deferred income taxes	1,596		1,522
Total liabilities	2,294		2,262
Shareholders’ equity
Share capital	6,308		6,730
Additional paid-in capital	148		259
Accumulated other comprehensive (loss) income	655		417
Retained earnings	4,794		4,716
Total shareholders' equity	11,905		12,122
Total liabilities and shareholders’ equity	$ 14,199		$ 14,384