Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unrealized foreign exchange (loss) gain on: Before tax amount
Translation of the net investment in U.S. subsidiaries	CAD (132)	CAD 671	CAD 287
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	150	(757)	(319)
Change in derivatives designated as cash flow hedges: Before tax amount
Realized loss (gain) on cash flow hedges recognized in income	10	7	(3)
Unrealized loss on cash flow hedges	(12)	(76)	(46)
Change in pension and other benefits actuarial gains and losses	(422)	1,058	(873)
Change in prior service pension and other benefit costs	(12)	1	(68)
Other comprehensive income (loss)	(418)	904	(1,022)
Unrealized foreign exchange (loss) gain on: Income tax recovery (expense)
Translation of the net investment in U.S. subsidiaries	0	0	0
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	(20)	100	42
Change in derivatives designated as cash flow hedges: Income tax recovery (expense)
Realized loss (gain) on cash flow hedges recognized in income	(2)	(2)	0
Unrealized loss on cash flow hedges	2	21	12
Change in pension and other benefits actuarial gains and losses	113	(281)	234
Change in prior service pension and other benefit costs	3	0	18
Other comprehensive income (loss)	96	(162)	306
Unrealized foreign exchange (loss) gain on: Net of tax amount
Translation of the net investment in U.S. subsidiaries	(132)	671	287
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	130	(657)	(277)
Change in derivatives designated as cash flow hedges: Net of tax amount
Realized loss on cash flow hedges recognized in income	8	5	(3)
Unrealized loss on cash flow hedges	(10)	(55)	(34)
Change in pension and other benefits actuarial gains and losses	(309)	777	(639)
Change in prior service pension and other benefit costs	(9)	1	(50)
Other comprehensive income	CAD (322)	CAD 742	CAD (716)