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Fair Value Measurements - Long-term Debt (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Sep. 30, 2018
Carrying amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 5,856.6 $ 4,199.4
Estimated fair value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 6,189.3 $ 4,150.3