Derivative Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Asset derivatives:
Derivative instruments, at fair value	$ 203,002	$ 304,237
Liability derivatives:
Derivative instruments, at fair value	18,353	75,562
Margin deposits received (remitted to) counterparties	(400)	(700)
Commodity derivatives
Asset derivatives:
Derivative instruments, at fair value	203,002	304,237
Derivative instruments subject to master netting agreements	(17,824)	(73,753)
Derivative instruments, net	185,178	230,484
Liability derivatives:
Derivative instruments, at fair value	18,353	75,562
Derivative instruments subject to master netting agreements	(17,824)	(73,753)
Margin deposits received (remitted to) counterparties	(428)	(736)
Derivative instruments, net	$ 101	$ 1,073

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be offset against an obligation to return collateral under a master netting arrangement.

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Fair value before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be offset against a right to receive collateral under a master netting arrangement.

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The amount of cash or securities placed with a broker or counterparty as security for a trading or derivatives securities position which was partially obtained with funds provided by the broker dealer.

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