Stock-Based Compensation - Summary of Activity Related to Employee PSUs (Detail) - Performance Share Unit Plan [Member] $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2021 CAD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Units, Outstanding as of December 31, 2020 | shares	390
Number of Units, Granted including DRIP | shares	91
Number of Units, Exercised | shares	(175)
Number of Units, Forfeited | shares	(26)
Number of Units, Transferred | shares	(5)
Number of Units, Outstanding as of December 31, 2020 | shares	285
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2020 | $ / shares	$ 46.87
Weighted Average Grant Date Fair Value, Granted including DRIP | $ / shares	52.25
Weighted Average Grant Date Fair Value, Exercised | $ / shares	48.12
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	47.82
Weighted Average Grant Date Fair Value, Transferred | $ / shares	47.18
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2020 | $ / shares	$ 47.74
Aggregate Intrinsic Value, Outstanding as of December 31, 2020 | $	$ 21
Aggregate Intrinsic Value, Granted including DRIP | $	5
Aggregate Intrinsic Value, Exercised | $	10
Aggregate Intrinsic Value, Forfeited | $	1
Aggregate Intrinsic Value, Transferred | $	0
Aggregate Intrinsic Value, Outstanding as of December 31, 2020 | $	$ 18