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Bank Revolving Credit Facility (Details) (USD $)
In Thousands
Jul. 31, 2011
Jan. 31, 2011
Bank Revolving Credit Facility    
Maximum borrowings $ 450,000 $ 470,336
Less outstanding balances and reserves:    
Borrowings 0 137,152
Letters of credit 67,896 63,418
Surety bonds 0 0
Reserve for retirement of debt 121,413 46,891
Available credit $ 260,691 $ 222,875