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Impairment fair value, ceiling test and transactions costs (Details 5) - USD ($) | 3 Months Ended
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Jun. 30, 2016 | Mar. 31, 2016
Exploration and production [Member] |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |
Fair value impairment | $ 900,000 | $ (1,400,000)
Fair value impairment after tax | 600,000 | $ (900,000)
Refining [Member] | Discontinued operations, held-for-sale or disposed by sale [Member] |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |
Fair value impairment | 251,900,000 |
Fair value impairment after tax | $ 156,700,000 |

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