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		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,373)	$ (2,756)	$ (2,103)	$ (2,814)
Other comprehensive income (loss) before reclassifications	805	(76)	502	(59)
Amounts reclassified from accumulated other comprehensive loss	29	41	62	82
Net other comprehensive (loss) income	834	(35)	564	23
AOC Income (Loss), Closing balance	(1,539)	(2,791)	(1,539)	(2,791)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(531)	112	(182)	112
Other comprehensive income (loss) before reclassifications	748	(2)	399	(2)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net other comprehensive (loss) income	748	(2)	399	(2)
AOC Income (Loss), Closing balance	217	110	217	110
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(3)	(21)	(4)	(40)
Other comprehensive income (loss) before reclassifications	0	(72)	0	(55)
Amounts reclassified from accumulated other comprehensive loss	1	2	2	4
Net other comprehensive (loss) income	1	(70)	2	(51)
AOC Income (Loss), Closing balance	(2)	(91)	(2)	(91)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(1,884)	(2,839)	(1,915)	(2,878)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	28	39	59	78
Net other comprehensive (loss) income	28	39	59	78
AOC Income (Loss), Closing balance	(1,856)	(2,800)	(1,856)	(2,800)
Equity Accounted Investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	45	(8)	(2)	(8)
Other comprehensive income (loss) before reclassifications	57	(2)	103	(2)
Amounts reclassified from accumulated other comprehensive loss	0	0	1	0
Net other comprehensive (loss) income	57	(2)	104	(2)
AOC Income (Loss), Closing balance	$ 102	$ (10)	$ 102	$ (10)