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Fair value measurements (Details 2) - USD ($) $ in Thousands | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Dec. 31, 2014
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage investment in common stock of mid-cap companies | 9.00% |  |  |  | 20.00% |
Percentage investment in common stock of small-cap companies | 7.00% |  |  |  | 18.00% |
Percentage investment in common stock of large-cap companies | 19.00% |  |  |  | 29.00% |
Percentage in fixed-income investments | 63.00% |  |  |  | 32.00% |
Percentage Investment in Target Date Investments | 1.00% |  |  |  |  |
Percentage investment in cash and cash equivalents | 1.00% |  |  |  | 1.00% |
Assets, fair value disclosure, nonrecurring |  |  | $ 10,800 | $ 1,100 |  |
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | $ 79,286 |  |  |  | $ 76,912 |
Liabilities: | 3,052 |  |  |  |  |
Fair value, measurements, recurring [Member] | Renewable Identification Number Obligations [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 3,052 |  |  |  |  |
Fair value, measurements, recurring [Member] | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,420 |  |  |  | 890 |
Fair value, measurements, recurring [Member] | Insurance contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 67,459 | [1] |  |  | 65,831 | [2]
Fair value, measurements, recurring [Member] | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 9,098 |  |  |  | 6,636 |
Fair value, measurements, recurring [Member] | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,309 |  |  |  | 3,555 |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  |  | 0 |
Liabilities: | 0 |  |  |  |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Renewable Identification Number Obligations [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 0 |  |  |  |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  |  | 0 |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 | [1] |  |  | 0 | [2]
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  |  | 0 |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  |  | 0 |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 79,286 |  |  |  | 76,912 |
Liabilities: | 3,052 |  |  |  |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Renewable Identification Number Obligations [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 3,052 |  |  |  |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,420 |  |  |  | 890 |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Insurance contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 67,459 | [1] |  |  | 65,831 | [2]
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 9,098 |  |  |  | 6,636 |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,309 |  |  |  | 3,555 |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  |  | 0 |
Liabilities: | 0 |  |  |  |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Renewable Identification Number Obligations [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 0 |  |  |  |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  |  | 0 |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Insurance contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 | [1] |  |  | 0 | [2]
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  |  | 0 |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | $ 0 |  |  |  | $ 0 |
[1] | The insurance contract invests approximately 9 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 19 percent in common stock of large-cap companies, 63 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.

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