Derivatives (Details 1) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 46,329	$ 61,247	$ (106,629)
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	121,789	103,105	122,010
Commodity derivative assets - noncurrent	87	1,906	4,844
Commodity derivative liabilities - current	58,183	33,277	29,764
Commodity derivative liabilities - noncurrent	5,863	1,134	454
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	103,184	144,541	97,454
Commodity derivative liabilities	(78,718)	(60,169)	(152,757)
Cash collateral	33,364		151,939
Cash collateral		(13,772)
Total	57,830	70,600	96,636
Commodity \| Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	98,902	137,119	89,552
Commodity derivative liabilities	(10,477)	(20,242)	(119,481)
Cash collateral	33,364
Cash collateral		(13,772)	151,939
Commodity derivative assets - current	121,789	103,105	122,010
Commodity \| Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	87	2,059	4,950
Commodity derivative liabilities	0	(153)	(106)
Cash collateral	0
Cash collateral		0	0
Commodity derivative assets - noncurrent	87	1,906	4,844
Commodity \| Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	4,133	5,233	2,943
Commodity derivative liabilities	(62,316)	(38,510)	(32,707)
Cash collateral	0		0
Cash collateral		0
Commodity derivative liabilities - current	(58,183)	(33,277)	(29,764)
Commodity \| Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	62	130	9
Commodity derivative liabilities	(5,925)	(1,264)	(463)
Cash collateral	0
Cash collateral		0	0
Commodity derivative liabilities - noncurrent	$ (5,863)	$ (1,134)	$ (454)

Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of after a year or beyond the normal operating cycle, if longer.

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The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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The amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

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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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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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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, and which are expected to be extinguished or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.

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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, and which are expected to be extinguished or otherwise disposed of after one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.

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