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Taxes and operating loss carryforwards (Details 4) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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State and local jurisdiction [Member] |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |
Operating loss carryforwards | $ 153.2 | $ 130.1
Exploration and production [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] |  |
Deferred tax assets, operating loss carryforwards | 1.9 | 2.6
Exploration and production [Member] | Domestic tax authority [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] |  |
Operating loss carryforwards |  | 4.4
Exploration and production [Member] | State and local jurisdiction [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] |  |
Operating loss carryforwards |  | $ 13.8
Exploration and production [Member] | Federal and State Tax Authorities [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] |  |
Operating loss carryforwards | $ 0.0 |

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