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Debt (Details 2) (USD $)
 6 Months Ended
Aug. 31, 2012
Feb. 29, 2012
Long-term debt    
Total long-term debt $ 175,000,000 $ 178,000,000
Less current maturities of long-term debt   (3,000,000)
Long-term debt, excluding current maturities 175,000,000 175,000,000
Limit on additional debt borrowings 237,600,000  
Fixed rate Senior Notes | Book value
    
Long-term debt    
Long-term debt 100,000,000  
Fixed rate Senior Notes | Fair Value, Level 2 | Discounted cash flow analysis
    
Long-term debt    
Long-term debt 104,560,000  
7.24% unsecured senior note payable
    
Long-term debt    
Face amount   15,000,000
Interest rate (as a percent)   7.24%
Interest rate (as a percent)   7.24%
Principal payments 3,000,000  
Frequency of principal payment Annual  
Total long-term debt   3,000,000
3.90% unsecured senior notes payable
    
Long-term debt    
Face amount 100,000,000  
Interest rate (as a percent) 3.90%  
Interest rate (as a percent) 3.90%  
Principal payments 20,000,000  
Frequency of principal payment Annual  
Total long-term debt 100,000,000 100,000,000
Unsecured floating interest rate 10 year Senior Notes
    
Long-term debt    
Face amount 75,000,000  
Interest rate (as a percent) 6.01%  
Debt instrument term (in years) 10  
Variable Rate Basis three-month LIBOR  
Basis spread on variable rate (as percent) 0.90%  
Total long-term debt $ 75,000,000 $ 75,000,000