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Borrowing Arrangements (Detail) - USD ($)
1 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2012
Jan. 31, 2016
Jan. 31, 2015
Debt Disclosure [Line Items]        
Unsecured revolving credit facility     $ 400,000,000  
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, expiration     May 31, 2020  
Line of credit facility, outstanding borrowings     $ 0 $ 0
Senior Notes | Senior Notes due 2025 [Member]        
Debt Disclosure [Line Items]        
Face Amount $ 300,000,000      
Stated interest rate 4.375%      
Maturity Date Jun. 15, 2025      
Senior Notes | Senior Notes due 2020 [Member]        
Debt Disclosure [Line Items]        
Face Amount $ 450,000,000      
Stated interest rate 3.125%      
Maturity Date Jun. 15, 2020      
Senior Notes | 2015 Senior Notes [Member]        
Debt Disclosure [Line Items]        
Proceeds from Debt, Net of Issuance Costs $ 742,000,000      
Unamortized Discount 1,700,000      
Debt Issuance Cost $ 6,300,000      
Debt redemption percentage of principle amount (as a percent) 101.00%      
Fair Value Disclosure     756,900,000  
Senior Notes | Senior Notes due 2017 [Member]        
Debt Disclosure [Line Items]        
Face Amount   $ 400,000,000    
Stated interest rate   1.95%    
Maturity Date   Dec. 15, 2017    
Senior Notes | Senior Notes due 2022 [Member]        
Debt Disclosure [Line Items]        
Face Amount   $ 350,000,000    
Stated interest rate   3.60%    
Maturity Date   Dec. 15, 2022    
Senior Notes | 2012 Senior Notes [Member]        
Debt Disclosure [Line Items]        
Proceeds from Debt, Net of Issuance Costs   $ 739,300,000    
Unamortized Discount   4,500,000    
Debt Issuance Cost   $ 6,100,000    
Debt redemption percentage of principle amount (as a percent)   101.00%    
Fair Value Disclosure     $ 738,000,000