Taxes and operating loss carryforwards (Details 3) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred tax liabilities, property, plant and equipment	$ 791,368,000	$ 773,160,000
Deferred tax assets, asset retirement obligation	8,694,000	7,894,000
Valuation allowance	8,990,000	8,852,000
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	78,900,000
Deferred tax assets, property, plant and equipment	38,100,000
Deferred tax liabilities, property, plant and equipment		270,000,000
Deferred tax assets, asset retirement obligation		26,400,000
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	208,200,000	0
State and Local Jurisdiction [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating loss carryforwards	116,200,000	114,300,000
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	201,400,000	5,900,000
Valuation allowance	$ 300,000	$ 253,000