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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance			CAD 4,796	CAD 5,610
Ending balance	CAD 4,669	CAD 3,992	4,669	3,992
Total [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	(1,471)	(2,133)	(1,477)	(2,219)
Other comprehensive income (loss) before reclassifications	2	(28)	(64)	(39)
Amounts reclassified from accumulated other comprehensive loss	36	48	108	145
Net current-period other comprehensive income	38	20	44	106
Ending balance	(1,433)	(2,113)	(1,433)	(2,113)
Foreign currency net of hedging activities [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	124	125	129	115
Other comprehensive income (loss) before reclassifications	2	6	(3)	16
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income	2	6	(3)	16
Ending balance	126	131	126	131
Derivatives and other [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	(157)	(77)	(102)	(52)
Other comprehensive income (loss) before reclassifications	(1)	(34)	(60)	(60)
Amounts reclassified from accumulated other comprehensive loss	2	2	6	3
Net current-period other comprehensive income	1	(32)	(54)	(57)
Ending balance	(156)	(109)	(156)	(109)
Pension and post-retirement defined benefit plans [Member]
Changes in Accumulated Other Comprehensive Loss by Component [Roll Forward]
Beginning balance	(1,438)	(2,181)	(1,504)	(2,282)
Other comprehensive income (loss) before reclassifications	1	0	(1)	5
Amounts reclassified from accumulated other comprehensive loss	34	46	102	142
Net current-period other comprehensive income	35	46	101	147
Ending balance	CAD (1,403)	CAD (2,135)	CAD (1,403)	CAD (2,135)