Condensed Consolidating Financial Information - Interim Condensed Consolidating Balance Sheets (Details) - CAD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 125	$ 338
Accounts receivable, net	699	687
Accounts receivable, inter-company	0	0
Short-term advances to affiliates	0	0
Materials and supplies	154	152
Other current assets	139	97
Total current assets	1,117	1,274
Long-term advances to affiliates	0	0
Investments	186	182
Investments in subsidiaries	0	0
Properties	17,234	17,016
Goodwill and intangible assets	192	187
Pension asset	1,507	1,407
Other assets	82	69
Deferred income taxes	0	0
Total assets	20,318	20,135
Current liabilities
Accounts payable and accrued liabilities	1,183	1,238
Accounts payable, inter-company	0	0
Short-term advances from affiliates	0	0
Long-term debt maturing within one year	756	746
Total current liabilities	1,939	1,984
Pension and other benefit liabilities	749	749
Long-term advances from affiliates	0	0
Other long-term liabilities	205	231
Long-term debt	7,601	7,413
Deferred income taxes	3,390	3,321
Total liabilities	13,884	13,698
Shareholders’ equity
Share capital	2,022	2,032
Additional paid-in capital	45	43
Accumulated other comprehensive (loss) income	(1,705)	(1,741)	$ (1,769)	$ (1,799)
Retained earnings	6,072	6,103
Total shareholders' equity	6,434	6,437	$ 5,038	$ 4,626
Total liabilities and shareholders’ equity	20,318	20,135
Consolidating Adjustments and Eliminations [Member]
Current assets
Cash and cash equivalents	0	0
Accounts receivable, net	0	0
Accounts receivable, inter-company	(458)	(465)
Short-term advances to affiliates	(6,118)	(6,500)
Materials and supplies	0	0
Other current assets	(84)	0
Total current assets	(6,660)	(6,965)
Long-term advances to affiliates	(1,178)	(1,000)
Investments	0	0
Investments in subsidiaries	(22,806)	(22,745)
Properties	0	0
Goodwill and intangible assets	0	0
Pension asset	0	0
Other assets	0	0
Deferred income taxes	(4)	(3)
Total assets	(30,648)	(30,713)
Current liabilities
Accounts payable and accrued liabilities	(84)	0
Accounts payable, inter-company	(458)	(465)
Short-term advances from affiliates	(6,118)	(6,500)
Long-term debt maturing within one year	0	0
Total current liabilities	(6,660)	(6,965)
Pension and other benefit liabilities	0	0
Long-term advances from affiliates	(1,178)	(1,000)
Other long-term liabilities	0	0
Long-term debt	0	0
Deferred income taxes	(4)	(3)
Total liabilities	(7,842)	(7,968)
Shareholders’ equity
Share capital	(7,345)	(7,767)
Additional paid-in capital	(1,909)	(1,902)
Accumulated other comprehensive (loss) income	1,158	1,325
Retained earnings	(14,710)	(14,401)
Total shareholders' equity	(22,806)	(22,745)
Total liabilities and shareholders’ equity	(30,648)	(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	104	97
Short-term advances to affiliates	0	500
Materials and supplies	0	0
Other current assets	0	0
Total current assets	104	597
Long-term advances to affiliates	1,090	590
Investments	0	0
Investments in subsidiaries	10,944	10,623
Properties	0	0
Goodwill and intangible assets	0	0
Pension asset	0	0
Other assets	0	0
Deferred income taxes	4	3
Total assets	12,142	11,813
Current liabilities
Accounts payable and accrued liabilities	102	82
Accounts payable, inter-company	4	3
Short-term advances from affiliates	5,602	5,291
Long-term debt maturing within one year	0	0
Total current liabilities	5,708	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	5,708	5,376
Shareholders’ equity
Share capital	2,022	2,032
Additional paid-in capital	45	43
Accumulated other comprehensive (loss) income	(1,705)	(1,741)
Retained earnings	6,072	6,103
Total shareholders' equity	6,434	6,437
Total liabilities and shareholders’ equity	12,142	11,813
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Current assets
Cash and cash equivalents	43	241
Accounts receivable, net	525	508
Accounts receivable, inter-company	149	153
Short-term advances to affiliates	1,313	1,004
Materials and supplies	121	120
Other current assets	80	31
Total current assets	2,231	2,057
Long-term advances to affiliates	0	0
Investments	27	27
Investments in subsidiaries	11,862	12,122
Properties	9,016	8,982
Goodwill and intangible assets	0	0
Pension asset	1,507	1,407
Other assets	73	56
Deferred income taxes	0	0
Total assets	24,716	24,651
Current liabilities
Accounts payable and accrued liabilities	886	844
Accounts payable, inter-company	305	309
Short-term advances from affiliates	508	1,185
Long-term debt maturing within one year	756	746
Total current liabilities	2,455	3,084
Pension and other benefit liabilities	670	672
Long-term advances from affiliates	1,178	1,000
Other long-term liabilities	93	108
Long-term debt	7,549	7,362
Deferred income taxes	1,827	1,802
Total liabilities	13,772	14,028
Shareholders’ equity
Share capital	1,038	1,037
Additional paid-in capital	1,648	1,643
Accumulated other comprehensive (loss) income	(1,705)	(1,742)
Retained earnings	9,963	9,685
Total shareholders' equity	10,944	10,623
Total liabilities and shareholders’ equity	24,716	24,651
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Current assets
Cash and cash equivalents	82	97
Accounts receivable, net	174	179
Accounts receivable, inter-company	205	215
Short-term advances to affiliates	4,805	4,996
Materials and supplies	33	32
Other current assets	143	66
Total current assets	5,442	5,585
Long-term advances to affiliates	88	410
Investments	159	155
Investments in subsidiaries	0	0
Properties	8,218	8,034
Goodwill and intangible assets	192	187
Pension asset	0	0
Other assets	9	13
Deferred income taxes	0	0
Total assets	14,108	14,384
Current liabilities
Accounts payable and accrued liabilities	279	312
Accounts payable, inter-company	149	153
Short-term advances from affiliates	8	24
Long-term debt maturing within one year	0	0
Total current liabilities	436	489
Pension and other benefit liabilities	79	77
Long-term advances from affiliates	0	0
Other long-term liabilities	112	123
Long-term debt	52	51
Deferred income taxes	1,567	1,522
Total liabilities	2,246	2,262
Shareholders’ equity
Share capital	6,307	6,730
Additional paid-in capital	261	259
Accumulated other comprehensive (loss) income	547	417
Retained earnings	4,747	4,716
Total shareholders' equity	11,862	12,122
Total liabilities and shareholders’ equity	$ 14,108	$ 14,384