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Summary of Significant Accounting Policies (Details 4) (USD $)
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2008
Mar. 31, 2009
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%        
Noncash write-down of natural gas and oil properties $ 0 $ 0 $ 620,000,000    
Noncash write-downs of natural gas and oil production properties, after tax     384,400,000    
Additional write-downs of its natural gas and oil properties that would have been recognized without effects of cash flow hedges         107,900,000
Additional write-downs of its natural gas and oil properties that would have been recognized without effects of cash flow hedges, after tax         66,900,000
Natural gas and oil properties not subject to amortization [Abstract]          
Acquisition 185,773,000        
Development 9,938,000        
Exploration 27,439,000        
Capitalized interest 9,312,000        
Total costs not subject to amortization 232,462,000 182,402,000 178,214,000    
Acquisition Costs, Period Cost 50,721,000 71,315,000 988,000 62,749,000  
Development Costs, Period Cost 9,689,000 156,000 2,000 91,000  
Exploration Costs, Period Cost 24,389,000 2,710,000 72,000 268,000  
Interest Costs, Capitalized During Period 3,539,000 3,096,000 44,000 2,633,000  
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost $ 88,338,000 $ 77,277,000 $ 1,106,000 $ 65,741,000  
Time period for evaluating the majority of costs not subject to amortization (in years) five years