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Stock-Based Compensation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Mar. 09, 2017
Mar. 08, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Percentage of vesting in first anniversary 30.00%        
Percentage of vesting in second anniversary 30.00%        
Percentage of vesting in third anniversary 40.00%        
Stock based compensation outstanding units 12,047        
Net stock-based compensation expense $ 10 $ 10 $ 8    
Recognized stock-based compensation expense in connection with the modification of outstanding equity awards   5      
Closing common stock price $ 55.80        
Stock Options, Weighted Average Grant Date Fair Value, Granted $ 12.76        
Stock Options [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Percentage of exercise price equal to market price on the date of grant 100.00%        
Vesting period (in years) 3 years        
Expiration period, years 10 years        
Unrecognized compensation cost $ 2        
Weighted-average period, years 1 year        
Intrinsic value of options exercised $ 16 8 2    
Cash received from exercises $ 21 $ 8 $ 4    
Stock Options, Weighted Average Grant Date Fair Value, Granted $ 12.76 $ 11.12 $ 15.68    
Restricted Stock Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting period (in years) 3 years        
Stock based compensation outstanding units [1] 876,000 732,000      
Unrecognized compensation cost $ 18        
Weighted-average period, years 1 year 3 months 19 days        
Estimated weighted average grant date fair value $ 51.03 $ 40.51 $ 45.85    
Intrinsic value of restricted stock units vested $ 7 $ 15 $ 14    
Restricted Stock          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting period (in years) 3 years        
Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Cash settled awards $ 1 $ 1      
2009 Long-Term Incentive Plan [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, authorized shares         8,854,000
2017 Long-Term Incentive Plan [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, authorized shares       5,375,000  
Supplemental 401(k) Plan [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Supplemental retirement savings plan, aggregate value of the accounts, shares 109,000 143,000      
Supplemental retirement savings plan aggregate value of the accounts paid in cash $ 2 $ 1      
[1] The number granted represents the number of shares issuable upon vesting of time-based restricted stock units and performance-based restricted stock units, assuming the Company performs at the target performance level in each year of the three-year performance period.