Condensed Consolidating Financial Information - Interim Condensed Consolidating Statements of Income (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Freight	$ 1,709	$ 1,598	$ 3,334	$ 3,161
Non-freight	41	45	78	85
Total revenues	1,750	1,643	3,412	3,246
Operating expenses
Compensation and benefits	351	345	725	645
Fuel	230	160	445	330
Materials	53	48	108	97
Equipment rents	33	37	66	73
Depreciation and amortization	172	165	342	331
Purchased services and other	284	277	559	555
Total operating expenses	1,123	1,032	2,245	2,031
Operating income	627	611	1,167	1,215
Less:
Other income and charges	52	(61)	103	(89)
Other components of net periodic benefit (recovery) cost	(95)	(68)	(191)	(135)
Net interest expense (income)	112	122	227	242
Income before income tax expense	558	618	1,028	1,197
Income tax expense (recovery)	122	138	244	286
Equity in net earnings of subsidiaries	0	0	0	0
Net income	436	480	784	911
Consolidating Adjustments and Eliminations [Member]
Revenues
Freight	0	0	0	0
Non-freight	(80)	(84)	(159)	(169)
Total revenues	(80)	(84)	(159)	(169)
Operating expenses
Compensation and benefits	0	1	2	2
Fuel	0	0	0	0
Materials	3	6	8	12
Equipment rents	0	0	0	0
Depreciation and amortization	0	0	0	0
Purchased services and other	(83)	(91)	(169)	(183)
Total operating expenses	(80)	(84)	(159)	(169)
Operating income	0	0	0	0
Less:
Other income and charges	0	0	0	0
Other components of net periodic benefit (recovery) cost	0	0	0	0
Net interest expense (income)	0	0	0	0
Income before income tax expense	0	0	0	0
Income tax expense (recovery)	0	0	0	0
Equity in net earnings of subsidiaries	(645)	(591)	(1,152)	(1,157)
Net income	(645)	(591)	(1,152)	(1,157)
CPRL (Parent Guarantor) [Member] | Reportable Legal Entities [Member]
Revenues
Freight	0	0	0	0
Non-freight	0	0	0	0
Total revenues	0	0	0	0
Operating expenses
Compensation and benefits	0	0	0	0
Fuel	0	0	0	0
Materials	0	0	0	0
Equipment rents	0	0	0	0
Depreciation and amortization	0	0	0	0
Purchased services and other	0	0	0	0
Total operating expenses	0	0	0	0
Operating income	0	0	0	0
Less:
Other income and charges	5	(5)	11	(25)
Other components of net periodic benefit (recovery) cost	0	0	0	0
Net interest expense (income)	(2)	(9)	6	(7)
Income before income tax expense	(3)	14	(17)	32
Income tax expense (recovery)	(1)	1	(1)	2
Equity in net earnings of subsidiaries	438	467	800	881
Net income	436	480	784	911
CPRC (Subsidiary Issuer) [Member] | Reportable Legal Entities [Member]
Revenues
Freight	1,196	1,129	2,351	2,218
Non-freight	31	34	58	66
Total revenues	1,227	1,163	2,409	2,284
Operating expenses
Compensation and benefits	237	234	494	426
Fuel	178	122	346	254
Materials	38	34	73	68
Equipment rents	30	39	61	75
Depreciation and amortization	105	108	209	217
Purchased services and other	205	210	423	418
Total operating expenses	793	747	1,606	1,458
Operating income	434	416	803	826
Less:
Other income and charges	79	(59)	127	(66)
Other components of net periodic benefit (recovery) cost	(96)	(69)	(192)	(137)
Net interest expense (income)	121	139	235	264
Income before income tax expense	330	405	633	765
Income tax expense (recovery)	99	62	185	160
Equity in net earnings of subsidiaries	207	124	352	276
Net income	438	467	800	881
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Revenues
Freight	513	469	983	943
Non-freight	90	95	179	188
Total revenues	603	564	1,162	1,131
Operating expenses
Compensation and benefits	114	110	229	217
Fuel	52	38	99	76
Materials	12	8	27	17
Equipment rents	3	(2)	5	(2)
Depreciation and amortization	67	57	133	114
Purchased services and other	162	158	305	320
Total operating expenses	410	369	798	742
Operating income	193	195	364	389
Less:
Other income and charges	(32)	3	(35)	2
Other components of net periodic benefit (recovery) cost	1	1	1	2
Net interest expense (income)	(7)	(8)	(14)	(15)
Income before income tax expense	231	199	412	400
Income tax expense (recovery)	24	75	60	124
Equity in net earnings of subsidiaries	0	0	0	0
Net income	$ 207	$ 124	$ 352	$ 276