Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in AOCL by Component (Details) - CAD CAD in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	CAD (1,799)	CAD (1,477)
Other comprehensive (loss) income before reclassifications	0	(40)
Amounts reclassified from accumulated other comprehensive income	30	36
Net current-period other comprehensive (loss) income	30	(4)
Closing balance	(1,769)	(1,481)
Foreign currency net of hedging activities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	127	129
Other comprehensive (loss) income before reclassifications	(2)	(4)
Amounts reclassified from accumulated other comprehensive income	0	0
Net current-period other comprehensive (loss) income	(2)	(4)
Closing balance	125	125
Derivatives and other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(104)	(102)
Other comprehensive (loss) income before reclassifications	2	(36)
Amounts reclassified from accumulated other comprehensive income	2	2
Net current-period other comprehensive (loss) income	4	(34)
Closing balance	(100)	(136)
Pension and post-retirement defined benefit plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(1,822)	(1,504)
Other comprehensive (loss) income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	28	34
Net current-period other comprehensive (loss) income	28	34
Closing balance	CAD (1,794)	CAD (1,470)