Condensed Consolidating Financial Information - Condensed Consolidating Statements of Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 664	$ 618	$ 724	$ 434	$ 545	$ 622	$ 436	$ 348	$ 2,440	$ 1,951	$ 2,405
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									37	(60)	24
Change in derivatives designated as cash flow hedges									10	38	19
Change in pension and post-retirement defined benefit plans									(661)	(449)	80
Other comprehensive income (loss) before income taxes									(614)	(471)	123
Income tax recovery (expense) on above items									135	169	(65)
Equity accounted investments									0	0	0
Other comprehensive (loss) income									(479)	(302)	58
Comprehensive income									1,961	1,649	2,463
Consolidating Adjustments and Eliminations
Condensed Financial Statements, Captions [Line Items]
Net income									(3,166)	(2,772)	(3,547)
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									0	0	0
Change in derivatives designated as cash flow hedges									0	0	0
Change in pension and post-retirement defined benefit plans									0	0	0
Other comprehensive income (loss) before income taxes									0	0	0
Income tax recovery (expense) on above items									0	0	0
Equity accounted investments									737	(121)	237
Other comprehensive (loss) income									737	(121)	237
Comprehensive income									(2,429)	(2,893)	(3,310)
CPRL (Parent Guarantor) | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Net income									2,440	1,951	2,405
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									0	0	0
Change in derivatives designated as cash flow hedges									0	0	0
Change in pension and post-retirement defined benefit plans									0	0	0
Other comprehensive income (loss) before income taxes									0	0	0
Income tax recovery (expense) on above items									0	0	0
Equity accounted investments									(479)	(302)	58
Other comprehensive (loss) income									(479)	(302)	58
Comprehensive income									1,961	1,649	2,463
CPRC (Subsidiary Issuer) | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Net income									2,430	1,969	2,367
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									288	(479)	318
Change in derivatives designated as cash flow hedges									10	38	19
Change in pension and post-retirement defined benefit plans									(651)	(455)	82
Other comprehensive income (loss) before income taxes									(353)	(896)	419
Income tax recovery (expense) on above items									132	171	(66)
Equity accounted investments									(258)	423	(295)
Other comprehensive (loss) income									(479)	(302)	58
Comprehensive income									1,951	1,667	2,425
Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Net income									736	803	1,180
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									(251)	419	(294)
Change in derivatives designated as cash flow hedges									0	0	0
Change in pension and post-retirement defined benefit plans									(10)	6	(2)
Other comprehensive income (loss) before income taxes									(261)	425	(296)
Income tax recovery (expense) on above items									3	(2)	1
Equity accounted investments									0	0	0
Other comprehensive (loss) income									(258)	423	(295)
Comprehensive income									$ 478	$ 1,226	$ 885