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Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013 | Mar. 31, 2013
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage investment in common stock of mid-cap companies | 29.00% |  | 29.00% |  | 29.00% |
Percentage investment in common stock of small-cap companies | 27.00% |  | 28.00% |  | 28.00% |
Percentage investment in common stock of large-cap companies | 28.00% |  | 28.00% |  | 28.00% |
Percentage in fixed-income and other investments | 16.00% |  | 15.00% |  | 15.00% |
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | $ 93,933 |  | $ 93,657 |  | $ 101,013 |
Liabilities, fair value disclosure | 12,186 |  | 7,483 |  | 12,445 |
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 12,186 |  | 7,483 |  | 7,987 |
Fair value, measurements, recurring [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  |  |  | 4,458 |
Fair value, measurements, recurring [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 20,267 |  | 19,227 |  | 31,281 |
Fair value, measurements, recurring [Member] | Insurance contract [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 63,269 | [1] | 62,370 | [2] | 53,334 | [2]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 7,989 |  | 8,193 |  | 8,879 |
Fair value, measurements, recurring [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 2,078 |  | 1,917 |  | 1,340 |
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 330 |  | 1,950 |  | 6,179 |
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] | Interest rate swap [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  |  |  | 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 | 93,933 |  | 93,657 |  | 101,013 |
Liabilities, fair value disclosure | 12,186 |  | 7,483 |  | 12,445 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 12,186 |  | 7,483 |  | 7,987 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  |  |  | 4,458 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 20,267 |  | 19,227 |  | 31,281 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 63,269 | [1] | 62,370 | [2] | 53,334 | [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 | 7,989 |  | 8,193 |  | 8,879 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 2,078 |  | 1,917 |  | 1,340 |
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 | 330 |  | 1,950 |  | 6,179 |
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] | Interest rate swap [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  |  |  | 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 29 percent in common stock of mid-cap companies, 27 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 16 percent in fixed-income investments.
[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=25499696&loc=d3e19207-110258 + Details
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