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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,450)	$ (2,009)	$ (2,522)	$ (2,043)
Other comprehensive income (loss) before reclassifications	(1)	1	1	(1)
Amounts reclassified from accumulated other comprehensive loss	35	16	105	52
Net other comprehensive (loss) income	34	17	106	51
AOC Income (Loss), Closing balance	(2,416)	(1,992)	(2,416)	(1,992)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	116	112	112	113
Other comprehensive income (loss) before reclassifications	(1)	1	3	0
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net other comprehensive (loss) income	(1)	1	3	0
AOC Income (Loss), Closing balance	115	113	115	113
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(52)	(58)	(54)	(62)
Other comprehensive income (loss) before reclassifications	0	0	(2)	0
Amounts reclassified from accumulated other comprehensive loss	2	1	6	5
Net other comprehensive (loss) income	2	1	4	5
AOC Income (Loss), Closing balance	(50)	(57)	(50)	(57)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,514)	(2,063)	(2,580)	(2,094)
Other comprehensive income (loss) before reclassifications	0	0	0	(1)
Amounts reclassified from accumulated other comprehensive loss	33	15	99	47
Net other comprehensive (loss) income	33	15	99	46
AOC Income (Loss), Closing balance	$ (2,481)	$ (2,048)	$ (2,481)	$ (2,048)