Components of Deferred Income Tax Liabilities and Assets (Detail) (CAD) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components Of Deferred Tax Assets And Liabilities [Line Items]
Depreciation and amortization	3,777	2,949	2,434
Successful drilling and land acquisitions	827	815	399
Pension and benefits	(438)	(376)	(717)
Site restoration	(304)	(287)	(284)
Capitalized interest	82	69	53
Other	(103)us-gaap_DeferredTaxLiabilitiesOther	(99)us-gaap_DeferredTaxLiabilitiesOther	39us-gaap_DeferredTaxLiabilitiesOther
Net long-term deferred income tax liabilities	3,841	3,071	1,924
LIFO inventory valuation	(201)us-gaap_DeferredTaxAssetsInventory	(450)us-gaap_DeferredTaxAssetsInventory	(478)us-gaap_DeferredTaxAssetsInventory
Other	(113)us-gaap_DeferredTaxAssetsOther	(109)us-gaap_DeferredTaxAssetsOther	(49)us-gaap_DeferredTaxAssetsOther
Net current deferred income tax assets	(314)us-gaap_DeferredTaxAssetsNetCurrent	(559)us-gaap_DeferredTaxAssetsNetCurrent	(527)us-gaap_DeferredTaxAssetsNetCurrent
Valuation allowance
Net deferred income tax liabilities	3,527us-gaap_DeferredTaxAssetsLiabilitiesNet	2,512us-gaap_DeferredTaxAssetsLiabilitiesNet	1,397us-gaap_DeferredTaxAssetsLiabilitiesNet

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The tax effect as of the balance sheet date of the amount of the estimated future tax effects arising from successful drilling and land acquisitions expensed for tax purposes, but capitalized for conformity with gaap.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from inventory.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards expected to be realized or consumed within one year or operating cycle, if longer.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from pension benefits.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from asset retirement obligations. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service of a tangible long-lived asset.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences from capitalized interest.

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Amount of deferred tax liability attributable to taxable temporary differences, net of deferred tax asset attributable to deductible temporary differences and carryforwards net of valuation allowances expected to be realized or consumed after one year (or the normal operating cycle, if longer).

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Amount of deferred tax liability attributable to taxable temporary differences not separately disclosed.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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