Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	$ (1,741)	$ (1,799)
Other comprehensive income (loss) before reclassifications	12	0
Amounts reclassified from accumulated other comprehensive loss	24	30
Net current-period other comprehensive income (loss)	36	30
AOC Income (Loss), Closing balance	(1,705)	(1,769)
Foreign currency net of hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	109	127
Other comprehensive income (loss) before reclassifications	0	(2)
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	0	(2)
AOC Income (Loss), Closing balance	109	125
Derivatives and other
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(89)	(104)
Other comprehensive income (loss) before reclassifications	13	2
Amounts reclassified from accumulated other comprehensive loss	2	2
Net current-period other comprehensive income (loss)	15	4
AOC Income (Loss), Closing balance	(74)	(100)
Pension and post-retirement defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(1,761)	(1,822)
Other comprehensive income (loss) before reclassifications	(1)	0
Amounts reclassified from accumulated other comprehensive loss	22	28
Net current-period other comprehensive income (loss)	21	28
AOC Income (Loss), Closing balance	$ (1,740)	$ (1,794)