Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unrealized foreign exchange gain (loss) on: Before tax amount
Translation of the net investment in U.S. subsidiaries	$ 419	$ (295)	$ (132)
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	(479)	319	150
Change in derivatives designated as cash flow hedges: Before tax amount
Realized loss on cash flow hedges recognized in income	10	25	10
Unrealized gain on cash flow hedges and other	28	(6)	(12)
Change in pension and other benefits actuarial gains and losses	(447)	84	(422)
Change in prior service pension and other benefit costs	(2)	(4)	(12)
Other comprehensive (loss) income	(471)	123	(418)
Unrealized foreign exchange gain (loss) on: Income tax (expense) recovery
Translation of the net investment in U.S. subsidiaries	0	0	0
Effect of hedge of net investment in foreign subsidiaries	64	(42)	(20)
Change in derivatives designated as cash flow hedges: Income tax (expense) recovery
Realized loss on cash flow hedges recognized in income	(3)	(6)	(2)
Unrealized gain on cash flow hedges and other	(8)	2	2
Change in pension and other benefits actuarial gains and losses	115	(20)	113
Change in prior service pension and other benefit costs	1	1	3
Other comprehensive (loss) income	169	(65)	96
Unrealized foreign exchange gain (loss) on: Net of tax amount
Translation of the net investment in U.S. subsidiaries	419	(295)	(132)
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	(415)	277	130
Change in derivatives designated as cash flow hedges: Net of tax amount
Realized loss on cash flow hedges recognized in income	7	19	8
Unrealized gain on cash flow hedges and other	20	(4)	(10)
Change in pension and other benefits actuarial gains and losses	(332)	64	(309)
Change in prior service pension and other benefit costs	(1)	(3)	(9)
Other comprehensive (loss) income	$ (302)	$ 58	$ (322)