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Debt (Narrative) (Details) - USD ($)
1 Months Ended 12 Months Ended
Mar. 31, 2017
Sep. 30, 2016
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Oct. 01, 2018
Sep. 27, 2018
Debt Disclosure [Abstract]              
Short-term debt, weighted average interest rate     5.00% 4.60%      
Cash paid for interest, net of amounts capitalized     $ 123,100,000 $ 125,900,000 $ 120,600,000    
Debt Instrument [Line Items]              
Short-term borrowings     54,300,000 144,000,000.0   $ 54,300,000  
Loss on extinguishment of debt     0 0 6,900,000    
Versum Financing              
Debt Instrument [Line Items]              
Loss on extinguishment of debt   $ 6,900,000     $ 6,900,000    
Foreign Line of Credit | Foreign Subsidiary Facility              
Debt Instrument [Line Items]              
Credit facility, amount borrowed and outstanding     7,000,000.0        
Revolving Credit Facility | 2018 Credit Agreement              
Debt Instrument [Line Items]              
Credit facility, amount borrowed and outstanding     $ 0        
Debt instrument, term 5 years            
Revolving Credit Facility | 2018 Credit Agreement | Maximum              
Debt Instrument [Line Items]              
Covenant, ratio of total debt to total capitalization     70.00%        
Revolving Credit Facility | 2017 Credit Agreement              
Debt Instrument [Line Items]              
Maximum borrowing capacity     $ 2,300,000,000.0       $ 2,500,000,000.0
Bank obligations              
Debt Instrument [Line Items]              
Short-term borrowings     $ 54,300,000 $ 144,000,000.0