Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	CAD (2,219)	CAD (1,503)
Other comprehensive income (loss) before reclassifications	544	(805)
Amounts reclassified from accumulated other comprehensive loss	198	89
Net current-period other comprehensive income (loss)	742	(716)	CAD 1,265
Closing balance	(1,477)	(2,219)	(1,503)
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	115	105
Other comprehensive income (loss) before reclassifications	14	10
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	14	10
Closing balance	129	115	105
Derivatives and Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(52)	(15)
Other comprehensive income (loss) before reclassifications	(55)	(34)
Amounts reclassified from accumulated other comprehensive loss	5	(3)
Net current-period other comprehensive income (loss)	(50)	(37)
Closing balance	(102)	(52)	(15)
Pension and Post-retirement Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(2,282)	(1,593)
Other comprehensive income (loss) before reclassifications	585	(781)
Amounts reclassified from accumulated other comprehensive loss	193	92
Net current-period other comprehensive income (loss)	778	(689)
Closing balance	CAD (1,504)	CAD (2,282)	CAD (1,593)