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(Additional Information) (Details) - Senior Notes - USD ($)
$ in Millions
Aug. 31, 2018
Feb. 28, 2018
Aug. 31, 2016
4.900% Senior Notes      
Debt Instrument [Line Items]      
Senior Notes, stated interest rate 4.90%    
Estimated fair value of senior notes $ 306.5    
5.625% Senior Notes      
Debt Instrument [Line Items]      
Senior Notes, stated interest rate 5.625%    
Estimated fair value of senior notes $ 415.7    
4.700% Senior Notes      
Debt Instrument [Line Items]      
Senior Notes, stated interest rate 4.70%    
Estimated fair value of senior notes $ 503.5    
3.950% Senior Notes      
Debt Instrument [Line Items]      
Senior Notes, stated interest rate 3.95% 3.95% 3.95%
Estimated fair value of senior notes $ 476.0