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Fair value measurements (Details 2) (USD $) | Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013
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Concentration risks, percentage [Abstract] |  |  |  |  |  |  |
Percentage investment in common stock of mid-cap companies | 21.00% |  | 29.00% |  | 29.00% |  |
Percentage investment in common stock of small-cap companies | 17.00% |  | 28.00% |  | 28.00% |  |
Percentage investment in common stock of large-cap companies | 29.00% |  | 28.00% |  | 28.00% |  |
Percentage in fixed-income and other investments | 32.00% |  | 15.00% |  | 15.00% |  |
Percentage investment in cash and cash equivalents | 1.00% |  |  |  |  |  |
Assets, fair value disclosure, nonrecurring |  |  |  |  |  |  | $ 9,700,000
Fair value, measurements, recurring [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 105,121,000 |  | 93,657,000 |  | 95,329,000 |  |
Liabilities, fair value disclosure | 44,000 |  | 7,483,000 |  | 9,889,000 |  |
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Liabilities, fair value disclosure | 44,000 |  | 7,483,000 |  | 9,889,000 |  |
Fair value, measurements, recurring [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 19,687,000 |  | 19,227,000 |  | 21,019,000 |  |
Fair value, measurements, recurring [Member] | Insurance contract [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 63,578,000 | [1] | 62,370,000 | [2] | 58,142,000 | [2] |
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 7,901,000 |  | 8,193,000 |  | 8,101,000 |  |
Fair value, measurements, recurring [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 2,374,000 |  | 1,917,000 |  | 1,923,000 |  |
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 11,581,000 |  | 1,950,000 |  | 6,144,000 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] | 0 | [2] |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 105,121,000 |  | 93,657,000 |  | 95,329,000 |  |
Liabilities, fair value disclosure | 44,000 |  | 7,483,000 |  | 9,889,000 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Liabilities, fair value disclosure | 44,000 |  | 7,483,000 |  | 9,889,000 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 19,687,000 |  | 19,227,000 |  | 21,019,000 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 63,578,000 | [1] | 62,370,000 | [2] | 58,142,000 | [2] |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 7,901,000 |  | 8,193,000 |  | 8,101,000 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 2,374,000 |  | 1,917,000 |  | 1,923,000 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 11,581,000 |  | 1,950,000 |  | 6,144,000 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] | 0 | [2] |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | $ 0 |  | $ 0 |  | $ 0 |  |
[1] | The insurance contract invests approximately 21 percent in common stock of mid-cap companies, 17 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.
[2] | The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Fair value of financial and nonfinancial obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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