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Oil and natural gas properties (Details) (USD $)
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Oil and Gas Property [Abstract]        
Discount rate used in calculating present value of future net cash flows from proved reserves   10.00%   10.00%
Write-down of oil and natural gas properties $ 0 $ 160,100,000 $ 0 $ 160,100,000
Write-down of oil and natural gas properties, after tax   100,900,000   100,900,000
Effects of cash flow hedges not considered in calculating ceiling limitation, amount   19,500,000   19,500,000
Effects of cash flow hedges not considered in calculating ceiling limitation amount, after tax   $ 12,300,000   $ 12,300,000