Derivative Instruments and Risk Management Activities (Details 6) (Commodity derivative, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Commodity derivative
Commodity derivative assets
Gross amounts of recognized assets	$ 656,696	$ 597,359
Gross amounts offset on balance sheet	(62,781)	(65,344)
Net amounts of assets on balance sheet	593,915	532,015
Commodity derivative liabilities
Gross amounts of recognized assets	1,713
Gross amounts offset on balance sheet	(2,348)
Net amounts of assets (liabilities) on balance sheet	$ (635)

Fair value of liability associated with derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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- Definition

Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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- Definition

Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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- Definition

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