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Proved reserves (Details 6) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 MBoe | Dec. 31, 2011 MBoe | Dec. 31, 2010 MBoe
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Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, energy | 69,131 | 79,460 | 82,404
Proved undeveloped reserves, energy | 11,355 | 18,191 | 25,195
Balance at end of year, energy | 80,486 | 97,651 | 107,599
Changes in proved and undeveloped reserves [Abstract] |  |  |
Change in PUD reserve | (6,800) |  |
Amount of PUD reserves converted into proved developed reserves | 3,900 |  |
Negative revision to PUD reserves | 10,300 |  |
New PUD reserves | 7,400 |  |
Drilling, completion and future development costs [Abstract] |  |  |
Drilling and completion capital | $ 58.4 |  |
Future development costs estimated to be spent to develop PUD reserves year one | 147.5 |  |
Future development costs estimated to be spent to develop PUD reserves year two | 24.3 |  |
Future development costs estimated to be spent to develop PUD reserves year three | $ 12.0 |  |
Oil (MBbls) |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, volume | 27,412 | 23,653 | 22,352
Proved undeveloped reserves, volume | 6,041 | 3,352 | 3,314
Balance at end of year, volume | 33,453 | 27,005 | 25,666
NGL (MBbls) |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, volume | 5,342 | 5,225 | 4,234
Proved undeveloped reserves, volume | 1,811 | 2,117 | 2,967
Balance at end of year, volume | 7,153 | 7,342 | 7,201
Natural Gas (MMcf) |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, volume | 218,259 | 303,495 | 334,911
Proved undeveloped reserves, volume | 21,019 | 76,332 | 113,486
Balance at end of year, volume | 239,278 | 379,827 | 448,397

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- Definition Drilling and completion capital required to convert PUD reserves into proved developed reserves. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year one. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year three. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year two. + References No definition available. + Details
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- Definition Amount of new PUD reserves. + References No definition available. + Details
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- Definition The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable. + References No definition available. + Details
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