Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	$ (2,043)	$ (1,741)
Other comprehensive income (loss) before reclassifications	(2)	12
Amounts reclassified from accumulated other comprehensive loss	18	24
Net current-period other comprehensive income (loss)	16	36
AOC Income (Loss), Closing balance	(2,027)	(1,705)
Foreign currency net of hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	113	109
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	0	0
AOC Income (Loss), Closing balance	113	109
Derivatives and other
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(62)	(89)
Other comprehensive income (loss) before reclassifications	(1)	13
Amounts reclassified from accumulated other comprehensive loss	2	2
Net current-period other comprehensive income (loss)	1	15
AOC Income (Loss), Closing balance	(61)	(74)
Pension and post-retirement defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(2,094)	(1,761)
Other comprehensive income (loss) before reclassifications	(1)	(1)
Amounts reclassified from accumulated other comprehensive loss	16	22
Net current-period other comprehensive income (loss)	15	21
AOC Income (Loss), Closing balance	$ (2,079)	$ (1,740)