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, 2020	Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,522)	$ (2,043)
Other comprehensive income (loss) before reclassifications	7	(2)
Amounts reclassified from accumulated other comprehensive loss	35	18
Net other comprehensive income	42	16
AOC Income (Loss), Closing balance	(2,480)	(2,027)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	112	113
Other comprehensive income (loss) before reclassifications	7	0
Amounts reclassified from accumulated other comprehensive loss	0	0
Net other comprehensive income	7	0
AOC Income (Loss), Closing balance	119	113
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(54)	(62)
Other comprehensive income (loss) before reclassifications	0	(1)
Amounts reclassified from accumulated other comprehensive loss	2	2
Net other comprehensive income	2	1
AOC Income (Loss), Closing balance	(52)	(61)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,580)	(2,094)
Other comprehensive income (loss) before reclassifications	0	(1)
Amounts reclassified from accumulated other comprehensive loss	33	16
Net other comprehensive income	33	15
AOC Income (Loss), Closing balance	$ (2,547)	$ (2,079)