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Hedging Instruments - Additional Information (Details)
oz in Thousands, $ in Millions
6 Months Ended
Jul. 31, 2019
USD ($)
oz
Jul. 31, 2012
USD ($)
2012 4.40% Series B Notes, due July 2012    
Derivative [Line Items]    
Debt instrument face amount | $   $ 250.0
Interest Rate Swap | 2012 Debt Issuance    
Derivative [Line Items]    
Unrealized loss on interest rate swaps | $ $ 16.7  
Interest Rate Swap | 2014 Debt Issuance    
Derivative [Line Items]    
Unrealized loss on interest rate swaps | $ $ 3.4  
Foreign Exchange Forward | Maximum    
Derivative [Line Items]    
Term of contract (months) 12 months  
Precious Metal Forward Contracts | Maximum    
Derivative [Line Items]    
Term of contract (months) 17 months  
Platinum    
Derivative [Line Items]    
Precious metal derivatives, ounces outstanding (ounces) | oz 26  
Silver    
Derivative [Line Items]    
Precious metal derivatives, ounces outstanding (ounces) | oz 431  
Gold    
Derivative [Line Items]    
Precious metal derivatives, ounces outstanding (ounces) | oz 55