Unrecognized Tax Benefits (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Unrecognized Tax Benefits [Line Items]
Balance as of January 1	$ 36	$ 78	$ 106
Additions for prior years' tax positions	1	9	2
Reductions for prior years' tax positions		(2)
Settlements with tax authorities	(2)	(49)	(30)
Balance as of December 31	$ 35	$ 36	$ 78