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, 2019 CAD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Units, Outstanding as of December 31, 2018 | shares	258
Number of Units, Granted including DRIP | shares	135
Number of Units, Exercised | shares	(4)
Number of Units, Forfeited | shares	(23)
Number of Units, Transferred | shares	138	[1]
Number of Units, Outstanding as of December 31, 2019 | shares	504
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2018 | $ / shares	$ 46.68
Weighted Average Grant Date Fair Value, Granted including DRIP | $ / shares	43.20
Weighted Average Grant Date Fair Value, Exercised | $ / shares	42.95
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	46.16
Weighted Average Grant Date Fair Value, Transferred | $ / shares	45.41	[1]
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2019 | $ / shares	$ 45.45
Aggregate Intrinsic Value, Outstanding as of December 31, 2018 | $	$ 11
Aggregate Intrinsic Value, Granted including DRIP | $	6
Aggregate Intrinsic Value, Exercised | $	0
Aggregate Intrinsic Value, Forfeited | $	(1)
Aggregate Intrinsic Value, Transferred | $	8	[1]
Aggregate Intrinsic Value, Outstanding as of December 31, 2019 | $	$ 28

[1] As the result of a reorganization of shared services functions, certain employees and their associated performance share units were transferred from TSI to TEC effective December 2019. The balance at December 31, 2019 is reflective of this transfer.