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Borrowings and Credit Arrangements - Convertible Notes and Related Equity Components (Details) - USD ($)
 12 Months Ended
Sep. 30, 2017
Jul. 01, 2017
Schedule Of Borrowings [Line Items]    
Notes principal amount   $ 100,000,000.0
2012 Notes    
Schedule Of Borrowings [Line Items]    
Notes principal amount $ 245,400,000  
Unamortized discount and issuance costs (1,900,000)  
Net carrying amount 243,500,000  
Equity component, net of taxes 24,100,000  
Two Thousand And Thirteen Notes [Member]    
Schedule Of Borrowings [Line Items]    
Notes principal amount 202,000,000  
Unamortized discount and issuance costs (1,800,000)  
Net carrying amount 241,000,000  
Equity component, net of taxes 71,800,000  
Principal accretion $ 40,800,000