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Schedule of Lines of Credit (Details) - USD ($)
$ in Millions
Jan. 31, 2017
Jan. 31, 2016
Line of Credit Facility [Line Items]    
Available $ 12,500 $ 15,000
Five Year Facility [Member]    
Line of Credit Facility [Line Items]    
Available 5,000 6,000
Drawn 0 0
Undrawn 5,000 6,000
Three Hundred And Sixty Four Day Facility [Member]    
Line of Credit Facility [Line Items]    
Available 7,500 9,000
Drawn 0 0
Undrawn 7,500 9,000
Drawn amount    
Line of Credit Facility [Line Items]    
Drawn 0 0
Unused lines of Credit [Member]    
Line of Credit Facility [Line Items]    
Undrawn $ 12,500 $ 15,000