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Fair value measurements (Details 2) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Mar. 31, 2013
Concentration risks, percentage [Abstract]      
Percentage investment in common stock of mid-cap companies 29.00% 29.00% 29.00%
Percentage investment in common stock of small-cap companies 27.00% 28.00% 28.00%
Percentage investment in common stock of large-cap companies 28.00% 28.00% 28.00%
Percentage in fixed-income and other investments 16.00% 15.00% 15.00%
Fair value, measurements, recurring [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure $ 93,933 $ 93,657 $ 101,013
Liabilities, fair value disclosure 12,186 7,483 12,445
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member]
      
Fair value measurements [Line Items]      
Liabilities, fair value disclosure 12,186 7,483 7,987
Fair value, measurements, recurring [Member] | Interest rate swap [Member]
      
Fair value measurements [Line Items]      
Liabilities, fair value disclosure     4,458
Fair value, measurements, recurring [Member] | Money market funds [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 20,267 19,227 31,281
Fair value, measurements, recurring [Member] | Insurance contract [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 63,269 [1] 62,370 [2] 53,334 [2]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 7,989 8,193 8,879
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 2,078 1,917 1,340
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 330 1,950 6,179
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 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments [Member]
      
Fair value measurements [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 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 [2]
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 1 [Member] | Commodity derivative instruments , assets [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 93,933 93,657 101,013
Liabilities, fair value disclosure 12,186 7,483 12,445
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member]
      
Fair value measurements [Line Items]      
Liabilities, fair value disclosure 12,186 7,483 7,987
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Interest rate swap [Member]
      
Fair value measurements [Line Items]      
Liabilities, fair value disclosure     4,458
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 20,267 19,227 31,281
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 63,269 [1] 62,370 [2] 53,334 [2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 7,989 8,193 8,879
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 2,078 1,917 1,340
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments , assets [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure 330 1,950 6,179
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 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments [Member]
      
Fair value measurements [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 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 [2]
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
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments , assets [Member]
      
Fair value measurements [Line Items]      
Assets, fair value disclosure $ 0 $ 0 $ 0
[1] The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 27 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 16 percent in fixed-income investments.
[2] The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.