Short-term Debt (Details) $ in Millions	6 Months Ended
	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)
Short-term Debt [Abstract]
Commercial paper borrowings outstanding	$ 373.0	$ 448.6
Weighted average interest rate of commercial paper outstanding	1.04%	0.57%
Current portion of long-term debt	$ 250.0	$ 0.0
Line of Credit [Member]
Line of Credit Facility [Line Items]
Borrowing capacity	$ 1,000.0
Maximum [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Maximum debt to total capital ratio required by debt covenants		0.60
Maximum ratio of EBITDA to interest expense under credit facility	3.0