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, 2019	Dec. 31, 2018	Dec. 31, 2017
Unrealized foreign exchange (loss) gain on: Before tax amount
Translation of the net investment in U.S. subsidiaries, before tax	$ (251)	$ 419	$ (295)
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries, before tax	288	(479)	319
Change in derivatives designated as cash flow hedges: Before tax amount
Realized loss on cash flow hedges recognized in income, before tax	10	38	19
Unrealized gain (loss) on cash flow hedges and other, before tax		28	(6)
Change in pension and other benefits actuarial gains and losses, before tax	(661)	(447)	84
Change in prior service pension and other benefit costs, before tax		(2)	(4)
Other comprehensive (loss) income: Before tax amount	(614)	(471)	123
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	(38)	64	(42)
Change in derivatives designated as cash flow hedges: Income tax (expense) recovery
Realized loss on cash flow hedges recognized in income, tax	(2)
Unrealized gain (loss) on cash flow hedges and other, tax		(8)	2
Change in pension and other benefits actuarial gains and losses, tax	175	115	(20)
Change in prior service pension and other benefit costs, tax		1	1
Other comprehensive (loss) income: Income tax (expense) recovery	135	169	(65)
Unrealized foreign exchange gain (loss) on: Net of tax amount
Translation of the net investment in U.S. subsidiaries, after tax	(251)	419	(295)
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries, after tax	250	(415)	277
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax		10	25
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax		(3)	(6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		7	19
Change in derivatives designated as cash flow hedges: Net of tax amount
Realized loss on cash flow hedges recognized in income, after tax	8
Unrealized gain (loss) on cash flow hedges and other, after tax		20	(4)
Change in pension and other benefits actuarial gains and losses, after tax	(486)	(332)	64
Change in prior service pension and other benefit costs, after tax		(1)	(3)
Other comprehensive (loss) income	$ (479)	$ (302)	$ 58