Condensed Consolidating Financial Information - Interim Condensed Consolidating Statements of Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 348	$ 431
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	(20)	5
Change in derivatives designated as cash flow hedges	21	5
Change in pension and post-retirement defined benefit plans	29	38
Other comprehensive income (loss) before income taxes	30	48
Income tax (expense) recovery on above items	6	(18)
Equity accounted investments	0	0
Other comprehensive income (loss)	36	30
Comprehensive income (loss)	384	461
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(507)	(566)
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	0	0
Change in derivatives designated as cash flow hedges	0	0
Change in pension and post-retirement defined benefit plans	0	0
Other comprehensive income (loss) before income taxes	0	0
Income tax (expense) recovery on above items	0	0
Equity accounted investments	(167)	9
Other comprehensive income (loss)	(167)	9
Comprehensive income (loss)	(674)	(557)
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	348	431
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	0	0
Change in derivatives designated as cash flow hedges	0	0
Change in pension and post-retirement defined benefit plans	0	0
Other comprehensive income (loss) before income taxes	0	0
Income tax (expense) recovery on above items	0	0
Equity accounted investments	36	30
Other comprehensive income (loss)	36	30
Comprehensive income (loss)	384	461
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	362	414
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	(150)	45
Change in derivatives designated as cash flow hedges	21	5
Change in pension and post-retirement defined benefit plans	28	36
Other comprehensive income (loss) before income taxes	(101)	86
Income tax (expense) recovery on above items	6	(17)
Equity accounted investments	131	(39)
Other comprehensive income (loss)	36	30
Comprehensive income (loss)	398	444
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	145	152
Net (loss) gain in foreign currency translation adjustments, net of hedging activities	130	(40)
Change in derivatives designated as cash flow hedges	0	0
Change in pension and post-retirement defined benefit plans	1	2
Other comprehensive income (loss) before income taxes	131	(38)
Income tax (expense) recovery on above items	0	(1)
Equity accounted investments	0	0
Other comprehensive income (loss)	131	(39)
Comprehensive income (loss)	$ 276	$ 113