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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,480)	$ (2,027)	$ (2,522)	$ (2,043)
Other comprehensive income (loss) before reclassifications	(5)	0	2	(2)
Amounts reclassified from accumulated other comprehensive loss	35	18	70	36
Net other comprehensive (loss) income	30	18	72	34
AOC Income (Loss), Closing balance	(2,450)	(2,009)	(2,450)	(2,009)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	119	113	112	113
Other comprehensive income (loss) before reclassifications	(3)	(1)	4	(1)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net other comprehensive (loss) income	(3)	(1)	4	(1)
AOC Income (Loss), Closing balance	116	112	116	112
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(52)	(61)	(54)	(62)
Other comprehensive income (loss) before reclassifications	(2)	1	(2)	0
Amounts reclassified from accumulated other comprehensive loss	2	2	4	4
Net other comprehensive (loss) income	0	3	2	4
AOC Income (Loss), Closing balance	(52)	(58)	(52)	(58)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,547)	(2,079)	(2,580)	(2,094)
Other comprehensive income (loss) before reclassifications	0	0	0	(1)
Amounts reclassified from accumulated other comprehensive loss	33	16	66	32
Net other comprehensive (loss) income	33	16	66	31
AOC Income (Loss), Closing balance	$ (2,514)	$ (2,063)	$ (2,514)	$ (2,063)