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Fair value measurements Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
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Concentration risks, percentage [Abstract] |  |  |  |
Percentage investment in common stock of mid-cap companies | 28.00% |  | 33.00% |
Percentage investment in common stock of small-cap companies | 28.00% |  | 34.00% |
Percentage investment in common stock of large-cap companies | 29.00% |  | 32.00% |
Percentage in fixed-income and other investments | 15.00% |  |  |
Percentage investment in cash and cash equivalents |  |  | 1.00% |
Fair value, measurements, recurring [Member] |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 101,443 |  | 187,659 |
Liabilities: | 6,255 |  | 18,863 |
Fair value, measurements, recurring [Member] | Interest rate derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Liabilities: | 6,255 |  | 4,762 |
Fair value, measurements, recurring [Member] | Commodity derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Liabilities: |  |  | 14,101 |
Fair value, measurements, recurring [Member] | Money market funds |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 24,240 |  | 97,500 |
Fair value, measurements, recurring [Member] | Auction rate securities |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: |  |  | 11,400 |
Fair value, measurements, recurring [Member] | Mortgage-backed securities |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 8,195 |  | 8,296 |
Fair value, measurements, recurring [Member] | U.S. Treasury securities |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 1,806 |  | 1,656 |
Fair value, measurements, recurring [Member] | Insurance investment contract |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 48,898 | [1] | 38,352 | [2]
Fair value, measurements, recurring [Member] | Commodity derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 18,304 |  | 30,455 |
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 |  | 0 |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest rate derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Liabilities: | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity derivative instruments |  |  |  |
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) | Auction rate securities |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: |  |  | 0 |
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] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract |  |  |  |
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) | Commodity derivative instruments |  |  |  |
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: | 101,443 |  | 187,659 |
Liabilities: | 6,255 |  | 18,863 |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Interest rate derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Liabilities: | 6,255 |  | 4,762 |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Commodity derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Liabilities: |  |  | 14,101 |
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: | 24,240 |  | 97,500 |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Auction rate securities |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: |  |  | 11,400 |
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: | 8,195 |  | 8,296 |
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: | 1,806 |  | 1,656 |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Insurance investment contract |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 48,898 | [1] | 38,352 | [2]
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Commodity derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 18,304 |  | 30,455 |
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 |  | 0 |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Interest rate derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Liabilities: | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Commodity derivative instruments |  |  |  |
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) | Auction rate securities |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: |  |  | 0 |
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 |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Insurance investment contract |  |  |  |
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) | Commodity derivative instruments |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |
Assets: | 0 |  | 0 |
[1] | The insurance investment 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.
[2] | The insurance investment contract invests approximately 33 percent in common stock of mid-cap companies, 34 percent in common stock of small-cap companies, 32 percent in common stock of large-cap companies and 1 percent in cash and cash equivalents.

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