Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Unrealized foreign exchange (loss) gain on: Before tax amount
Translation of the net investment in U.S. subsidiaries, before tax	$ (118)	$ (251)	$ 419
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries, before tax	136	288	(479)
Realized loss on derivatives designated as cash flow hedges recognized in income, before tax	9	10	10
Unrealized gain on cash flow hedges and other, before tax			28
Change in pension and other benefits actuarial gains and losses, before tax	(403)	(661)	(447)
Change in prior service pension and other benefit costs, before tax	(4)		(2)
Other comprehensive (loss) income: Before tax amount	(380)	(614)	(471)
Unrealized foreign exchange gain (loss) on: Income tax (expense) recovery
Translation of the net investment in U.S. subsidiaries, tax	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, tax	(18)	(38)	64
Realized loss on cash flow hedges recognized in income, tax	(3)	(2)	(3)
Unrealized gain on cash flow hedges and other, tax			(8)
Change in pension and other benefits actuarial gains and losses, tax	108	175	115
Change in prior service pension and other benefit costs, tax	1		1
Other comprehensive (loss) income: Income tax (expense) recovery	88	135	169
Unrealized foreign exchange gain (loss) on: Net of tax amount
Translation of the net investment in U.S. subsidiaries, after tax	(118)	(251)	419
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries, after tax	118	250	(415)
Realized loss on derivatives designated as cash flow hedges recognized in income, after tax	6	8	7
Unrealized gain (loss) on cash flow hedges and other, after tax			(20)
Change in pension and other benefits actuarial gains and losses, after tax	(295)	(486)	(332)
Change in prior service pension and other benefit costs, after tax	(3)		(1)
Other comprehensive income (loss)	$ (292)	$ (479)	$ (302)