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Stock-Based Compensation - Stock options (Details) - Stock options - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2018
Mar. 31, 2017
Weighted-average assumptions utilized in determining the fair value of options      
Risk-free interest rate (as a percent)   2.20% 2.00%
Expected stock volatility (as a percent)   36.10% 39.40%
Dividend yield (as a percent)   4.80% 3.40%
Expected term (in years)   6 years 5 years 6 months
Options      
Options outstanding at the beginning of the period (in shares) 3,755 3,278  
Granted (in shares) 7 667  
Exercised (in shares) (41) (224)  
Option outstanding at the end of the period (in shares) 3,721 3,721  
Vested and expected to vest at the end of the period (in shares) 3,721 3,721  
Exercisable at the end of the period (in shares) 2,358 2,358  
Weighted-Average Exercise Price      
Outstanding at the beginning of the period (in dollars per share) $ 57.82 $ 56.41  
Granted (in dollars per share) 71.57 58.66  
Exercised (in dollars per share) 30.66 34.27  
Outstanding at the end of the period (in dollars per share) 58.15 58.15  
Vested and expected to vest at the end of the period (in dollars per share) 58.15 58.15  
Exercisable at the end of the period (in dollars per share) $ 55.64 $ 55.64  
Weighted- Average Remaining Contractual Term      
Outstanding at the end of the period 6 years 2 months 27 days    
Vested and expected to vest at the end of the period 6 years 2 months 27 days    
Weighted-average remaining life of exercisable stock options 4 years 9 months    
Aggregate Intrinsic Value      
Outstanding at the end of the period (in dollars) $ 40.1 $ 40.1  
Options vested or expected to vest, aggregate intrinsic value (in dollars) 40.1 40.1  
Exercisable at the end of the period (in dollars) $ 31.1 $ 31.1