Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD CAD in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	CAD (1,740)	CAD (1,471)	CAD (1,799)	CAD (1,477)
Other comprehensive loss before reclassifications	(5)	2	(15)	(64)
Amounts reclassified from accumulated other comprehensive loss	30	36	99	108
Net current-period other comprehensive (loss) income	25	38	84	44
AOC Income (Loss), Closing balance	(1,715)	(1,433)	(1,715)	(1,433)
Foreign currency net of hedging activities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	124	124	127	129
Other comprehensive loss before reclassifications	(5)	2	(8)	(3)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive (loss) income	(5)	2	(8)	(3)
AOC Income (Loss), Closing balance	119	126	119	126
Derivatives and other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(97)	(157)	(104)	(102)
Other comprehensive loss before reclassifications	0	(1)	(7)	(60)
Amounts reclassified from accumulated other comprehensive loss	2	2	16	6
Net current-period other comprehensive (loss) income	2	1	9	(54)
AOC Income (Loss), Closing balance	(95)	(156)	(95)	(156)
Pension and post-retirement defined benefit plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(1,767)	(1,438)	(1,822)	(1,504)
Other comprehensive loss before reclassifications	0	1	0	(1)
Amounts reclassified from accumulated other comprehensive loss	28	34	83	102
Net current-period other comprehensive (loss) income	28	35	83	101
AOC Income (Loss), Closing balance	CAD (1,739)	CAD (1,403)	CAD (1,739)	CAD (1,403)