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, 2018 CAD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Units, Outstanding as of December 31, 2017 | shares	133
Number of Units, Granted including DRIP | shares	130
Number of Units, Exercised | shares	(4)
Number of Units, Forfeited | shares	(1)
Number of Units, Outstanding as of December 31, 2018 | shares	258
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2017 | $ / shares	$ 45.11
Weighted Average Grant Date Fair Value, Granted including DRIP | $ / shares	47.98
Weighted Average Grant Date Fair Value, Exercised | $ / shares	38.85
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	45.41
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2018 | $ / shares	$ 46.68
Aggregate Intrinsic Value, Outstanding as of December 31, 2017 | $	$ 6
Aggregate Intrinsic Value, Granted including DRIP | $	6
Aggregate Intrinsic Value, Exercised | $	(1)
Aggregate Intrinsic Value, Forfeited | $	0
Aggregate Intrinsic Value, Outstanding as of December 31, 2018 | $	$ 11