Condensed Consolidating Financial Information - Interim Condensed Consolidating Statements of Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 436	$ 480	$ 784	$ 911
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	(16)	14	(36)	19
Change in derivatives designated as cash flow hedges	14	4	35	9
Change in pension and post-retirement defined benefit plans	29	37	58	75
Other comprehensive (loss) income before income taxes	27	55	57	103
Income tax recovery (expense) on above items	5	(26)	11	(44)
Equity accounted investments	0	0	0	0
Other comprehensive income (loss)	32	29	68	59
Comprehensive income	468	509	852	970
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(645)	(591)	(1,152)	(1,157)
Net (loss) gain 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 (loss) income before income taxes	0	0	0	0
Income tax recovery (expense) on above items	0	0	0	0
Equity accounted investments	(140)	73	(307)	82
Other comprehensive income (loss)	(140)	73	(307)	82
Comprehensive income	(785)	(518)	(1,459)	(1,075)
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	436	480	784	911
Net (loss) gain 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 (loss) income before income taxes	0	0	0	0
Income tax recovery (expense) on above items	0	0	0	0
Equity accounted investments	32	29	68	59
Other comprehensive income (loss)	32	29	68	59
Comprehensive income	468	509	852	970
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	438	467	800	881
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	(123)	117	(273)	162
Change in derivatives designated as cash flow hedges	14	4	35	9
Change in pension and post-retirement defined benefit plans	27	36	55	72
Other comprehensive (loss) income before income taxes	(82)	157	(183)	243
Income tax recovery (expense) on above items	6	(26)	12	(43)
Equity accounted investments	108	(102)	239	(141)
Other comprehensive income (loss)	32	29	68	59
Comprehensive income	470	496	868	940
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	207	124	352	276
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	107	(103)	237	(143)
Change in derivatives designated as cash flow hedges	0	0	0	0
Change in pension and post-retirement defined benefit plans	2	1	3	3
Other comprehensive (loss) income before income taxes	109	(102)	240	(140)
Income tax recovery (expense) on above items	(1)	0	(1)	(1)
Equity accounted investments	0	0	0	0
Other comprehensive income (loss)	108	(102)	239	(141)
Comprehensive income	$ 315	$ 22	$ 591	$ 135