Fair value measurements Fair value measurements (Details 2) (USD $)
In Thousands, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2011

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:

Liabilities:

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:

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:

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:

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:

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:

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:

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:

^[1]

^[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:

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:

Liabilities:

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:

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:

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:

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:

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:

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:

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:

^[1]

^[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:

[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.