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External Debt and Financing Arrangements - Summary of Outstanding Notes (Detail) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Senior Notes [Member]    
Debt Instrument [Line Items]    
Net Carrying Value $ 1,786.5 $ 2,184.3
3.000% Senior Notes [Member]    
Debt Instrument [Line Items]    
Principal Amount $ 400.0  
Issuance Date 2015-06  
Maturity Date 2020-06  
Net Carrying Value   399.7
4.000% Senior Notes [Member]    
Debt Instrument [Line Items]    
Principal Amount $ 500.0  
Issuance Date 2015-06  
Maturity Date 2025-06  
Net Carrying Value $ 496.4 495.8
4.000% Senior Notes [Member] | 2018 Senior Notes [Member]    
Debt Instrument [Line Items]    
Principal Amount $ 600.0  
Issuance Date 2018-09  
Maturity Date 2023-09  
Net Carrying Value $ 596.8 596.1
3.250% Senior Notes [Member]    
Debt Instrument [Line Items]    
Principal Amount $ 700.0  
Issuance Date 2019-09  
Maturity Date 2029-09  
Net Carrying Value $ 693.3 $ 692.7