Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	$ (1,673)	$ (1,740)	$ (1,741)	$ (1,799)
Other comprehensive income (loss) before reclassifications	(2)	(5)	19	(15)
Amounts reclassified from accumulated other comprehensive loss	21	30	68	99
Net current-period other comprehensive income (loss)	19	25	87	84
AOC Income (Loss), Closing balance	(1,654)	(1,715)	(1,654)	(1,715)
Foreign currency net of hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	110	124	109	127
Other comprehensive income (loss) before reclassifications	(1)	(5)	0	(8)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income (loss)	(1)	(5)	0	(8)
AOC Income (Loss), Closing balance	109	119	109	119
Derivatives and other
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(64)	(97)	(89)	(104)
Other comprehensive income (loss) before reclassifications	(2)	0	19	(7)
Amounts reclassified from accumulated other comprehensive loss	2	2	6	16
Net current-period other comprehensive income (loss)	0	2	25	9
AOC Income (Loss), Closing balance	(64)	(95)	(64)	(95)
Pension and post-retirement defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(1,719)	(1,767)	(1,761)	(1,822)
Other comprehensive income (loss) before reclassifications	1	0	0	0
Amounts reclassified from accumulated other comprehensive loss	19	28	62	83
Net current-period other comprehensive income (loss)	20	28	62	83
AOC Income (Loss), Closing balance	$ (1,699)	$ (1,739)	$ (1,699)	$ (1,739)