Goodwill And Other Intangible Assets (Details) CAD in Millions	12 Months Ended
Dec. 31, 2017 CAD $ / bbl	Dec. 31, 2016 CAD
Changes in intangible assets other than goodwill [abstract]
Balance at beginning of period	CAD 3,075.0	CAD 3,079.0
Amortization	(3.0)	(4.0)
Disposals	(13.0)
Additions	2.0
Balance at end of period	CAD 3,061.0	CAD 3,075.0
After tax discount rate applied to cash flow projections	8.00%	8.00%
Projected West Texas Intermediate price prices for 2018 \| $ / bbl	61.00
Projected West Texas Intermediate price prices for 2019 \| $ / bbl	68.60
Projected West Texas Intermediate price prices for 2020 \| $ / bbl	76.65
Average growth rate from 2021 - 2023	4.00%
Average growth rate after 2023	2.00%
Inflation rate	2.00%
Goodwill impairment	CAD 0.0
Minimum
Changes in intangible assets other than goodwill [abstract]
After tax discount rate applied to cash flow projections	10.00%	10.00%
Cash flow forecast period	P20Y
Maximum
Changes in intangible assets other than goodwill [abstract]
After tax discount rate applied to cash flow projections	12.00%	15.00%
Cash flow forecast period	P50Y
Oil Sands
Changes in intangible assets other than goodwill [abstract]
Goodwill impairment	CAD 0.0
Oil Sands \| Goodwill
Changes in intangible assets other than goodwill [abstract]
Balance at beginning of period	2,752.0	CAD 2,752.0
Balance at end of period	2,752.0	2,752.0
Refining and Marketing \| Goodwill
Changes in intangible assets other than goodwill [abstract]
Balance at beginning of period	148.0	148.0
Disposals	(8.0)
Balance at end of period	140.0	148.0
Refining and Marketing \| Brand Name
Changes in intangible assets other than goodwill [abstract]
Balance at beginning of period	166.0	166.0
Disposals	(4.0)
Balance at end of period	162.0	166.0
Refining and Marketing \| Customer Lists
Changes in intangible assets other than goodwill [abstract]
Balance at beginning of period	9.0	13.0
Amortization	(3.0)	(4.0)
Disposals	(1.0)
Additions	2.0
Balance at end of period	CAD 7.0	CAD 9.0

The amount of additions to intangible assets other than goodwill, other than those acquired through business combinations. [Refer: Business combinations [member]; Intangible assets other than goodwill]

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The amount of amortisation of intangible assets other than goodwill. [Refer: Depreciation and amortisation expense; Intangible assets other than goodwill]

+ References

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The discount rate applied to cash flow projections for a cash-generating unit (group of units). [Refer: Cash-generating units [member]]

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The decrease in intangible assets other than goodwill resulting from disposals. [Refer: Intangible assets other than goodwill]

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The explanation of the period over which management has projected cash flows based on financial budgets/forecasts approved by management including, when a period greater than five years is used for a cash-generating unit (group of units), an explanation of why that longer period is justified. [Refer: Cash-generating units [member]]

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The amount of impairment loss recognised in profit or loss for goodwill. [Refer: Impairment loss recognised in profit or loss; Goodwill]

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The amount of intangible assets and goodwill held by the entity. [Refer: Goodwill; Intangible assets other than goodwill]

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Represents the average growth percentage after four years in oil price per barrel used as a base for the cash flow projections.

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Represents the average growth percentage after six years in oil price per barrel used as a base for the cash flow projections.

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Represents the inflation rate of revenue and operating costs used to calculate the recoverable amount for the cash generating units goodwill impairment test.

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Represents the oil price per barrel used as a base for the cash flow projections after three years.

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Represents the oil price per barrel used as a base for the cash flow projections after two years.

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Represents the oil price per barrel used as a base for the cash flow projections for next year.

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