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
	Mar. 31, 2022	Mar. 31, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,103)	$ (2,814)
Other comprehensive income (loss) before reclassifications	(303)	17
Amounts reclassified from accumulated other comprehensive loss	33	41
Net other comprehensive (loss) income	(270)	58
AOC Income (Loss), Closing balance	(2,373)	(2,756)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(182)	112
Other comprehensive income (loss) before reclassifications	(349)	0
Amounts reclassified from accumulated other comprehensive loss	0	0
Net other comprehensive (loss) income	(349)	0
AOC Income (Loss), Closing balance	(531)	112
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(4)	(40)
Other comprehensive income (loss) before reclassifications	0	17
Amounts reclassified from accumulated other comprehensive loss	1	2
Net other comprehensive (loss) income	1	19
AOC Income (Loss), Closing balance	(3)	(21)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(1,915)	(2,878)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	31	39
Net other comprehensive (loss) income	31	39
AOC Income (Loss), Closing balance	(1,884)	(2,839)
Equity Accounted Investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2)	(8)
Other comprehensive income (loss) before reclassifications	46	0
Amounts reclassified from accumulated other comprehensive loss	1	0
Net other comprehensive (loss) income	47	0
AOC Income (Loss), Closing balance	$ 45	$ (8)