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Fair value measurements (Details 2) (USD $)
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Concentration risks, percentage [Abstract]        
Percentage investment in common stock of mid-cap companies 21.00% 29.00% 29.00%  
Percentage investment in common stock of small-cap companies 17.00% 28.00% 28.00%  
Percentage investment in common stock of large-cap companies 29.00% 28.00% 28.00%  
Percentage in fixed-income and other investments 32.00% 15.00% 15.00%  
Percentage investment in cash and cash equivalents 1.00%      
Assets, fair value disclosure, nonrecurring       $ 9,700,000
Fair value, measurements, recurring [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 105,121,000 93,657,000 95,329,000  
Liabilities, fair value disclosure 44,000 7,483,000 9,889,000  
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member]
        
Fair value measurements [Line Items]        
Liabilities, fair value disclosure 44,000 7,483,000 9,889,000  
Fair value, measurements, recurring [Member] | Money market funds [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 19,687,000 19,227,000 21,019,000  
Fair value, measurements, recurring [Member] | Insurance contract [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 63,578,000 [1] 62,370,000 [2] 58,142,000 [2]  
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 7,901,000 8,193,000 8,101,000  
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 2,374,000 1,917,000 1,923,000  
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 11,581,000 1,950,000 6,144,000  
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] | 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 105,121,000 93,657,000 95,329,000  
Liabilities, fair value disclosure 44,000 7,483,000 9,889,000  
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member]
        
Fair value measurements [Line Items]        
Liabilities, fair value disclosure 44,000 7,483,000 9,889,000  
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 19,687,000 19,227,000 21,019,000  
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 63,578,000 [1] 62,370,000 [2] 58,142,000 [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 7,901,000 8,193,000 8,101,000  
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
        
Fair value measurements [Line Items]        
Assets, fair value disclosure 2,374,000 1,917,000 1,923,000  
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 11,581,000 1,950,000 6,144,000  
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] | 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 21 percent in common stock of mid-cap companies, 17 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.
[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.