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Oil and natural gas properties (Details 6) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Natural gas and oil properties [Abstract]            
Discount rate used in calculating present value of future net cash flows from proved reserves (in hundredths)     10.00%      
NYMEX oil price 94.71 94.97   94.71    
Henry Hub natural gas price 2.76 2.83   2.76    
Write-downs of oil and natural gas properties $ 231,700,000 $ 160,100,000 $ 0 $ 391,800,000 $ 0  
Noncash write-downs of oil and natural gas properties, after tax 145,900,000 100,900,000   246,800,000    
Additional write-downs of its oil and natural gas properties that would have been recognized without effects of cash flow hedges 20,800,000 19,500,000   20,800,000    
Additional write-downs of its oil and natural gas properties that would have been recognized without effects of cash flow hedges, after tax 13,100,000 12,300,000   13,100,000    
Oil and natural gas properties not subject to amortization [Abstract]            
Acquisition     93,758,000      
Development     14,824,000      
Exploration     14,547,000      
Capitalized interest     1,740,000      
Total costs not subject to amortization 191,794,000   124,869,000 191,794,000 232,462,000  
Acquisition costs, period cost     1,514,000 23,588,000 28,543,000 40,113,000
Development costs, period cost     12,622,000 1,633,000 271,000 298,000
Exploration costs, period cost     9,952,000 4,346,000 198,000 51,000
Capitalized interest costs, period costs     340,000 418,000 410,000 572,000
Capitalized costs of unproved properties excluded from amortization, period cost     $ 24,428,000 $ 29,985,000 $ 29,422,000 $ 41,034,000
Time period for evaluating majority of costs not subject to amortization     5 years