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Stock-based Compensation Plans (Narrative) (Details)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
$ / shares
shares
Sep. 30, 2016
$ / shares
shares
Sep. 30, 2015
$ / shares
shares
Sep. 30, 2014
$ / shares
Dec. 14, 2011
shares
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount   238,826 867,065 1,165,414    
Weighted-average recognition period, Compensation expense related to stock awards not yet vested   1 year 7 months 6 days        
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 8.0 $ 8.0        
Number of shares authorized under a Plan           20,500,000
Shares available for future grants under a Plan 7,204,369 7,204,369        
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period   10 years        
Share-based Compensation, Expected Forfeiture Rate   2.00%        
Employee Stock Option [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   3 years        
Phantom Share Units (PSUs) [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   3 years        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period   199,260 302,875 289,524    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value | $ / shares $ 11.36 $ 11.36 $ 9.60 $ 8.49 $ 6.22  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value, Amount Per Share | $ / shares $ 12.80 $ 12.80        
Share-based compensation liability | $ $ 3.0 $ 3.0        
Employee Stock [Member]            
Number of shares authorized under a Plan 5,800,000.0 5,800,000.0        
Shares available for future grants under a Plan 2,901,604 2,901,604        
Price for shares offered under ESPP, as a percentage of closing price on the first or last day of the offering period 85.00%          
Performance Shares [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period   263,410 542,212      
Restricted Stock Units (RSUs) [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   3 years        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period   343,860 360,255 459,659    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value | $ / shares $ 11.23 $ 11.23 $ 9.34 $ 8.45 $ 5.04  
Minimum [Member] | Performance Shares [Member]            
Performance Factor   0        
Maximum [Member] | Performance Shares [Member]            
Performance Factor   2