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Fair value measurements (Details 2) (USD $) | Jun. 30, 2013 | Dec. 31, 2012 | Jun. 30, 2012
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage investment in common stock of mid-cap companies | 28.00% |  | 28.00% |  | 28.00% |
Percentage investment in common stock of small-cap companies | 28.00% |  | 28.00% |  | 28.00% |
Percentage investment in common stock of large-cap companies | 28.00% |  | 29.00% |  | 29.00% |
Percentage in fixed-income and other investments | 16.00% |  | 15.00% |  | 15.00% |
Assets, fair value disclosure, nonrecurring | $ 9,700,000 |  |  |  | $ 2,500,000 |
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 105,157,000 |  | 101,443,000 |  | 125,917,000 |
Liabilities, fair value disclosure | 1,388,000 |  | 6,255,000 |  | 8,000,000 |
Fair value, measurements, recurring [Member] | Commodity derivative instruments, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 1,388,000 |  |  |  | 1,037,000 |
Fair value, measurements, recurring [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  | 6,255,000 |  | 6,963,000 |
Fair value, measurements, recurring [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 29,902,000 |  | 24,240,000 |  | 21,054,000 |
Fair value, measurements, recurring [Member] | Insurance contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 54,039,000 | [1] | 48,898,000 | [2] | 45,959,000 | [2]
Fair value, measurements, recurring [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 8,052,000 |  | 8,195,000 |  | 8,253,000 |
Fair value, measurements, recurring [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 1,920,000 |  | 1,806,000 |  | 1,994,000 |
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 11,244,000 |  | 18,304,000 |  | 48,657,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [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, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  |  |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [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, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] | 0 | [2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [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, assets and liabilities measured on recurring and nonrecurring basis [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, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 105,157,000 |  | 101,443,000 |  | 125,917,000 |
Liabilities, fair value disclosure | 1,388,000 |  | 6,255,000 |  | 8,000,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 1,388,000 |  |  |  | 1,037,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  | 6,255,000 |  | 6,963,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 29,902,000 |  | 24,240,000 |  | 21,054,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 54,039,000 | [1] | 48,898,000 | [2] | 45,959,000 | [2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 8,052,000 |  | 8,195,000 |  | 8,253,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 1,920,000 |  | 1,806,000 |  | 1,994,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 11,244,000 |  | 18,304,000 |  | 48,657,000 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [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, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  |  |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure |  |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [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, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] | 0 | [2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [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, assets and liabilities measured on recurring and nonrecurring basis [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, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | $ 0 |  | $ 0 |  | $ 0 |
[1] | The insurance contract invests approximately 28 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 16 percent in fixed-income and other investments.
[2] | The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other 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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