Financial Instruments - Summary of Changes in Inputs to Option Pricing Model, Resulted in Unrealized Gains (Losses) Impacting Earnings Before Income Tax (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019 CAD ($) $ / bbl
WCS Forward Prices [Member]
Disclosure Of Sensitivity Of Fair Value Measurement To Changes In Unobservable Inputs Liabilities [Line Items]
Sensitivity Price Range | $ / bbl	5.00
Increase in volatility	$ (136)
Decrease in volatility	$ 94
WTI Option Volatility [Member]
Disclosure Of Sensitivity Of Fair Value Measurement To Changes In Unobservable Inputs Liabilities [Line Items]
Sensitivity Range	5.00%
Increase in volatility	$ (61)
Decrease in volatility	$ 58
Foreign Exchange Volatility Rate [Member]
Disclosure Of Sensitivity Of Fair Value Measurement To Changes In Unobservable Inputs Liabilities [Line Items]
Sensitivity Range	5.00%
Increase in volatility	$ 2
Decrease in volatility	$ (14)