Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	$ (1,705)	$ (1,769)	$ (1,741)	$ (1,799)
Other comprehensive income (loss) before reclassifications	9	(10)	21	(10)
Amounts reclassified from accumulated other comprehensive loss	23	39	47	69
Net current-period other comprehensive income (loss)	32	29	68	59
AOC Income (Loss), Closing balance	(1,673)	(1,740)	(1,673)	(1,740)
Foreign currency net of hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	109	125	109	127
Other comprehensive income (loss) before reclassifications	1	(1)	1	(3)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income (loss)	1	(1)	1	(3)
AOC Income (Loss), Closing balance	110	124	110	124
Derivatives and other
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(74)	(100)	(89)	(104)
Other comprehensive income (loss) before reclassifications	8	(9)	21	(7)
Amounts reclassified from accumulated other comprehensive loss	2	12	4	14
Net current-period other comprehensive income (loss)	10	3	25	7
AOC Income (Loss), Closing balance	(64)	(97)	(64)	(97)
Pension and post-retirement defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(1,740)	(1,794)	(1,761)	(1,822)
Other comprehensive income (loss) before reclassifications	0	0	(1)	0
Amounts reclassified from accumulated other comprehensive loss	21	27	43	55
Net current-period other comprehensive income (loss)	21	27	42	55
AOC Income (Loss), Closing balance	$ (1,719)	$ (1,767)	$ (1,719)	$ (1,767)