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Fair value measurements (Details 2) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Concentration risks, percentage [Abstract]      
Percentage investment in common stock of mid-cap companies 28.00% 33.00% 34.00%
Percentage investment in common stock of small-cap companies 28.00% 34.00% 33.00%
Percentage investment in common stock of large-cap companies 29.00% 32.00% 32.00%
Percentage in fixed-income and other investments 15.00%    
Percentage investment in cash and cash equivalents   1.00% 1.00%
Fair value, measurements, recurring [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure 103,288 187,659 165,568
Liabilities, fair value disclosure 9,934 18,863 6,676
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 2,155 14,101 3,185
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 7,779 4,762 3,491
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 21,816 97,500 56,194
Fair value, measurements, recurring [Member] | Insurance investment contract [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure 48,384 [1] 38,352 [2] 33,591 [3]
Fair value, measurements, recurring [Member] | Auction rate securities [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure   11,400 11,400
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 8,564 8,296 8,570
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 1,805 1,656 1,444
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 22,719 30,455 54,369
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 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 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 investment contract [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure 0 [1] 0 [2] 0 [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Auction rate securities [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure   0 0
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 103,288 187,659 165,568
Liabilities, fair value disclosure 9,934 18,863 6,676
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 2,155 14,101 3,185
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 7,779 4,762 3,491
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 21,816 97,500 56,194
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance investment contract [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure 48,384 [1] 38,352 [2] 33,591 [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Auction rate securities [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure   11,400 11,400
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 8,564 8,296 8,570
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 1,805 1,656 1,444
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 22,719 30,455 54,369
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 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 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 investment contract [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure 0 [1] 0 [2] 0 [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Auction rate securities [Member]
      
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]      
Assets, fair value disclosure   0 0
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 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.
[3] The insurance investment contract invests approximately 34 percent in common stock of mid-cap companies, 33 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.