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Debt and Credit Agreements (Details 3) (Revolving Credit Facility, USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
quarter
Dec. 31, 2011
Dec. 31, 2010
May 31, 2012
Debt        
Number of trailing quarters used to calculate annual coverage ratio 4      
Line of credit, outstanding $ 325.0 $ 188.0    
Availability under the credit facility 574.0      
Weighted-average effective interest rates (as a percent) 3.00% 4.10% 3.80%  
Weighted-average interest rate (as a percent) 2.20% 4.90%    
Available Line of Credit $ 1,700     $ 900
Greater than
        
Debt        
Commitment fee percentage 0.375%      
Current ratio 1.0      
Asset coverage ratio 1.75      
Percentage of the entity's total indebtedness to its borrowing base under the credit facility under the first level 25.00%      
Percentage of the entity's total indebtedness to its borrowing base under the credit facility under the second level 50.00%      
Percentage of the entity's total indebtedness to its borrowing base under the credit facility under the third level 75.00%      
Percentage of the entity's total indebtedness to its borrowing base under the credit facility under the fourth level 90.00%      
Less than or equal to
        
Debt        
Commitment fee percentage 0.50%      
Eurodollar Loans
        
Debt        
Description of variable rate basis Euro-Dollars (LIBOR)      
ABR Loans
        
Debt        
Description of variable rate basis Base Rate (Prime)      
Less than 25 percent | Eurodollar Loans
        
Debt        
Margin (as a percent) 1.50%      
Less than 25 percent | ABR Loans
        
Debt        
Margin (as a percent) 0.50%      
Greater than equal to 25 percent but less than 50 percent | Eurodollar Loans
        
Debt        
Margin (as a percent) 1.75%      
Greater than equal to 25 percent but less than 50 percent | ABR Loans
        
Debt        
Margin (as a percent) 0.75%      
Greater than equal to 50 percent but less than 75 percent | Eurodollar Loans
        
Debt        
Margin (as a percent) 2.00%      
Greater than equal to 50 percent but less than 75 percent | ABR Loans
        
Debt        
Margin (as a percent) 1.00%      
Greater than equal to 75 percent but less than 90 percent | Eurodollar Loans
        
Debt        
Margin (as a percent) 2.25%      
Greater than equal to 75 percent but less than 90 percent | ABR Loans
        
Debt        
Margin (as a percent) 1.25%      
Greater than and equal to 90 percent | Eurodollar Loans
        
Debt        
Margin (as a percent) 2.50%      
Greater than and equal to 90 percent | ABR Loans
        
Debt        
Margin (as a percent) 1.50%