Taxes and operating loss carryforwards (Details 4) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
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