Condensed Consolidating Financial Information - Interim Condensed Consolidating Statements of Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Net income	$ 434	$ 348
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	16	(20)
Change in derivatives designated as cash flow hedges	2	21
Change in pension and post-retirement defined benefit plans	20	29
Other comprehensive income (loss) before income taxes	38	30
Income tax (expense) recovery on above items	(22)	6
Equity accounted investments	0	0
Other comprehensive income (loss)	16	36
Comprehensive income	450	384
Consolidating Adjustments and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(552)	(507)
Net gain (loss) 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	87	(167)
Other comprehensive income (loss)	87	(167)
Comprehensive income	(465)	(674)
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	434	348
Net gain (loss) 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	16	36
Other comprehensive income (loss)	16	36
Comprehensive income	450	384
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	428	362
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	120	(150)
Change in derivatives designated as cash flow hedges	2	21
Change in pension and post-retirement defined benefit plans	19	28
Other comprehensive income (loss) before income taxes	141	(101)
Income tax (expense) recovery on above items	(22)	6
Equity accounted investments	(103)	131
Other comprehensive income (loss)	16	36
Comprehensive income	444	398
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	124	145
Net gain (loss) in foreign currency translation adjustments, net of hedging activities	(104)	130
Change in derivatives designated as cash flow hedges	0	0
Change in pension and post-retirement defined benefit plans	1	1
Other comprehensive income (loss) before income taxes	(103)	131
Income tax (expense) recovery on above items	0	0
Equity accounted investments	0	0
Other comprehensive income (loss)	(103)	131
Comprehensive income	$ 21	$ 276