Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD CAD in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	CAD (1,769)	CAD (1,481)	CAD (1,799)	CAD (1,477)
Other comprehensive (loss) income before reclassifications	(10)	(26)	(10)	(66)
Amounts reclassified from accumulated other comprehensive loss	39	36	69	72
Net current-period other comprehensive (loss) income	29	10	59	6
Closing balance	(1,740)	(1,471)	(1,740)	(1,471)
Foreign currency net of hedging activities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	125	125	127	129
Other comprehensive (loss) income before reclassifications	(1)	(1)	(3)	(5)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive (loss) income	(1)	(1)	(3)	(5)
Closing balance	124	124	124	124
Derivatives and other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(100)	(136)	(104)	(102)
Other comprehensive (loss) income before reclassifications	(9)	(23)	(7)	(59)
Amounts reclassified from accumulated other comprehensive loss	12	2	14	4
Net current-period other comprehensive (loss) income	3	(21)	7	(55)
Closing balance	(97)	(157)	(97)	(157)
Pension and post-retirement defined benefit plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(1,794)	(1,470)	(1,822)	(1,504)
Other comprehensive (loss) income before reclassifications	0	(2)	0	(2)
Amounts reclassified from accumulated other comprehensive loss	27	34	55	68
Net current-period other comprehensive (loss) income	27	32	55	66
Closing balance	CAD (1,767)	CAD (1,438)	CAD (1,767)	CAD (1,438)