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Fair Value Measurements (Reconciliation of Changes in Fair Value of Level 3 Derivative Instruments) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance	$ (5,050)		$ (10,486)
Included in earnings			67,105
Included in other comprehensive income
Settlements			4,831
Transfers in and out of Level 3	5,050	[1]
Balance			61,450
Change in unrealized losses included in earnings relating to derivatives still held at June 30			$ 71,936
[1] During the first nine months of 2012, the inputs used to value the Company's commodity derivative instruments were directly or indirectly observable and those contracts were transferred to Level 2.

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- Definition

Amount of unrealized gains and losses (net), included in the income statement for the period, from certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments) and which the entity has or had based its calculation of fair market value on unobservable inputs (Level 3).

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- Definition

Amount of gain (loss) recognized in earnings, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

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Amount of gain (loss) recognized in other comprehensive income, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

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Settlements that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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This element represents [net] transfers in to and out of assets measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

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This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

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