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, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	CAD (1,477)	CAD (2,219)
Other comprehensive (loss) income before reclassifications	(468)	544
Amounts reclassified from accumulated other comprehensive loss	146	198
Net current-period other comprehensive income (loss)	(322)	742	CAD (716)
Closing balance	(1,799)	(1,477)	(2,219)
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	129	115
Other comprehensive (loss) income before reclassifications	(2)	14
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	(2)	14
Closing balance	127	129	115
Derivatives and Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(102)	(52)
Other comprehensive (loss) income before reclassifications	(10)	(55)
Amounts reclassified from accumulated other comprehensive loss	8	5
Net current-period other comprehensive income (loss)	(2)	(50)
Closing balance	(104)	(102)	(52)
Pension and Post-retirement Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Opening balance	(1,504)	(2,282)
Other comprehensive (loss) income before reclassifications	(456)	585
Amounts reclassified from accumulated other comprehensive loss	138	193
Net current-period other comprehensive income (loss)	(318)	778
Closing balance	CAD (1,822)	CAD (1,504)	CAD (2,282)