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Borrowing Arrangements (Detail) - USD ($)
1 Months Ended 12 Months Ended
Jul. 31, 2017
Jun. 30, 2017
Jun. 30, 2015
Dec. 31, 2012
Jan. 31, 2018
Jan. 31, 2017
Debt Disclosure [Line Items]            
Unsecured revolving credit facility         $ 400,000,000.0  
Unsecured revolving credit facility, option to request an increase in the amount of the credit facility up to         100,000,000.0  
Line of credit facility, outstanding borrowings         0 $ 0
Senior Notes | 3.5% Notes due 2027            
Debt Disclosure [Line Items]            
Face amount   $ 500,000,000.0        
Stated interest rate   3.50%        
Unamortized discount   $ 3,100,000        
Debt issuance costs   4,900,000        
Proceeds from debt, net of issuance costs   $ 492,000,000.0        
Debt redemption percentage of principle amount (as a percent)   101.00%        
Fair value disclosure         485,600,000  
Senior Notes | Senior Notes due 2017            
Debt Disclosure [Line Items]            
Face amount       $ 400,000,000.0    
Stated interest rate       1.95%    
Debt redemption price $ 400,900,000          
Repayments of debt $ 401,800,000          
Senior Notes | Senior Notes due 2020            
Debt Disclosure [Line Items]            
Face amount     $ 450,000,000.0      
Stated interest rate     3.125%      
Senior Notes | Senior Notes due 2025            
Debt Disclosure [Line Items]            
Face amount     $ 300,000,000.0      
Stated interest rate     4.375%      
Senior Notes | 2015 Senior Notes            
Debt Disclosure [Line Items]            
Unamortized discount     $ 1,700,000      
Debt issuance costs     6,300,000      
Proceeds from debt, net of issuance costs     $ 742,000,000.0      
Debt redemption percentage of principle amount (as a percent)     101.00%      
Fair value disclosure         763,800,000  
Senior Notes | Senior Notes due 2022            
Debt Disclosure [Line Items]            
Face amount       $ 350,000,000.0    
Stated interest rate       3.60%    
Senior Notes | 2012 Senior Notes            
Debt Disclosure [Line Items]            
Unamortized discount       $ 4,500,000    
Debt issuance costs       6,100,000    
Proceeds from debt, net of issuance costs       $ 739,300,000    
Fair value disclosure         $ 354,400,000