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Fair value measurements (Details 2) - USD ($) $ in Thousands | Jun. 30, 2015 | Dec. 31, 2014 | Jun. 30, 2014
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage investment in common stock of mid-cap companies | 20.00% |  | 20.00% |  | 21.00% |
Percentage investment in common stock of small-cap companies | 18.00% |  | 18.00% |  | 18.00% |
Percentage investment in common stock of large-cap companies | 28.00% |  | 29.00% |  | 29.00% |
Percentage in fixed-income and other investments | 32.00% |  | 32.00% |  | 31.00% |
Percentage Investment in Target Date Investments | 1.00% |  |  |  |  |
Percentage investment in cash and cash equivalents | 1.00% |  | 1.00% |  | 1.00% |
Assets, Fair Value Disclosure, Nonrecurring | $ 1,100 |  |  |  |  |
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 94,967 |  | $ 92,160 |  | $ 90,651 |
Liabilities, fair value disclosure | 538 |  |  |  |  |
Fair value, measurements, recurring [Member] | Renewable Identification Number Obligations [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 538 |  |  |  |  |
Fair value, measurements, recurring [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 16,358 |  | 16,138 |  | 16,031 |
Fair value, measurements, recurring [Member] | Insurance contract [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 68,187 | [1] | 65,831 | [2] | 64,449 | [3]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 8,073 |  | 6,636 |  | 8,075 |
Fair value, measurements, recurring [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 2,349 |  | 3,555 |  | 2,096 |
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 |  |  |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Renewable Identification Number Obligations [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 | [3]
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 2 [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 94,967 |  | 92,160 |  | 90,651 |
Liabilities, fair value disclosure | 538 |  |  |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Renewable Identification Number Obligations [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 538 |  |  |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 16,358 |  | 16,138 |  | 16,031 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 68,187 | [1] | 65,831 | [2] | 64,449 | [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 8,073 |  | 6,636 |  | 8,075 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 2,349 |  | 3,555 |  | 2,096 |
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 |  |  |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Renewable Identification Number Obligations [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 | [3]
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 |
[1] | The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies, 32 percent in fixed-income investments, 1 percent in target date investments and 1 percent in cash equivalents.
[2] | The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 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.
[3] | The insurance contract invests approximately 21 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 31 percent in fixed-income investments and 1 percent in cash equivalents.

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- Definition The percentage of insurance contracts in fixed-income and other investments. + References No definition available. + Details
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