Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in Accumulated Other Comprehensive Loss (AOCL) by Component (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,791)	$ (2,450)	$ (2,814)	$ (2,522)
Other comprehensive income (loss) before reclassifications	107	(1)	48	1
Amounts reclassified from accumulated other comprehensive loss	41	35	123	105
Net other comprehensive (loss) income	148	34	171	106
AOC Income (Loss), Closing balance	(2,643)	(2,416)	(2,643)	(2,416)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	110	116	112	112
Other comprehensive income (loss) before reclassifications	6	(1)	4	3
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net other comprehensive (loss) income	6	(1)	4	3
AOC Income (Loss), Closing balance	116	115	116	115
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(101)	(52)	(48)	(54)
Other comprehensive income (loss) before reclassifications	101	0	44	(2)
Amounts reclassified from accumulated other comprehensive loss	2	2	6	6
Net other comprehensive (loss) income	103	2	50	4
AOC Income (Loss), Closing balance	2	(50)	2	(50)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,800)	(2,514)	(2,878)	(2,580)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	39	33	117	99
Net other comprehensive (loss) income	39	33	117	99
AOC Income (Loss), Closing balance	$ (2,761)	$ (2,481)	$ (2,761)	$ (2,481)