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Fair Value Measurements - Long-term Debt (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Sep. 30, 2016
Mar. 31, 2016
Carrying amount      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt $ 4,238.9 $ 3,832.3 $ 3,639.0
Estimated fair value      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt $ 4,255.0 $ 4,052.3 $ 3,775.3