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, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized foreign exchange gain (loss) on: Before tax amount
Translation of the net investment in U.S. subsidiaries	CAD (295)	CAD (132)	CAD 671
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	319	150	(757)
Change in derivatives designated as cash flow hedges: Before tax amount
Realized loss on cash flow hedges recognized in income	25	10	7
Unrealized loss on cash flow hedges	(6)	(12)	(76)
Change in pension and other benefits actuarial gains and losses	84	(422)	1,058
Change in prior service pension and other benefit costs	(4)	(12)	1
Comprehensive income (loss)	123	(418)	904
Unrealized foreign exchange gain (loss) 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	(42)	(20)	100
Change in derivatives designated as cash flow hedges: Income tax recovery (expense)
Realized loss on cash flow hedges recognized in income	(6)	(2)	(2)
Unrealized loss on cash flow hedges	2	2	21
Change in pension and other benefits actuarial gains and losses	(20)	113	(281)
Change in prior service pension and other benefit costs	1	3	0
Comprehensive income (loss)	(65)	96	(162)
Unrealized foreign exchange gain (loss) on: Net of tax amount
Translation of the net investment in U.S. subsidiaries	(295)	(132)	671
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	277	130	(657)
Change in derivatives designated as cash flow hedges: Net of tax amount
Realized loss on cash flow hedges recognized in income	19	8	5
Unrealized loss on cash flow hedges	(4)	(10)	(55)
Change in pension and other benefits actuarial gains and losses	64	(309)	777
Change in prior service pension and other benefit costs	(3)	(9)	1
Other comprehensive income (loss)	CAD 58	CAD (322)	CAD 742