Condensed Consolidating Financial Information - Interim Condensed Consolidating Statements of Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 622	$ 510	$ 1,406	$ 1,421
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	12	19	(24)	38
Change in derivatives designated as cash flow hedges	1	2	36	11
Change in pension and post-retirement defined benefit plans	28	38	86	113
Other comprehensive income (loss) before income taxes	41	59	98	162
Income tax recovery (expense) on above items	(22)	(34)	(11)	(78)
Equity accounted investments	0	0	0	0
Other comprehensive income (loss)	19	25	87	84
Comprehensive income	641	535	1,493	1,505
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(815)	(641)	(1,967)	(1,798)
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	0	0	0	0
Change in derivatives designated as cash flow hedges	0	0	0	0
Change in pension and post-retirement defined benefit plans	0	0	0	0
Other comprehensive income (loss) before income taxes	0	0	0	0
Income tax recovery (expense) on above items	0	0	0	0
Equity accounted investments	64	134	(243)	216
Other comprehensive income (loss)	64	134	(243)	216
Comprehensive income	(751)	(507)	(2,210)	(1,582)
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	622	510	1,406	1,421
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	0	0	0	0
Change in derivatives designated as cash flow hedges	0	0	0	0
Change in pension and post-retirement defined benefit plans	0	0	0	0
Other comprehensive income (loss) before income taxes	0	0	0	0
Income tax recovery (expense) on above items	0	0	0	0
Equity accounted investments	19	25	87	84
Other comprehensive income (loss)	19	25	87	84
Comprehensive income	641	535	1,493	1,505
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	615	505	1,415	1,386
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	96	180	(177)	342
Change in derivatives designated as cash flow hedges	1	2	36	11
Change in pension and post-retirement defined benefit plans	27	36	82	108
Other comprehensive income (loss) before income taxes	124	218	(59)	461
Income tax recovery (expense) on above items	(22)	(34)	(10)	(77)
Equity accounted investments	(83)	(159)	156	(300)
Other comprehensive income (loss)	19	25	87	84
Comprehensive income	634	530	1,502	1,470
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	200	136	552	412
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	(84)	(161)	153	(304)
Change in derivatives designated as cash flow hedges	0	0	0	0
Change in pension and post-retirement defined benefit plans	1	2	4	5
Other comprehensive income (loss) before income taxes	(83)	(159)	157	(299)
Income tax recovery (expense) on above items	0	0	(1)	(1)
Equity accounted investments	0	0	0	0
Other comprehensive income (loss)	(83)	(159)	156	(300)
Comprehensive income	$ 117	$ (23)	$ 708	$ 112