Financial Instruments (Schedule of Interest Rate Swaps and Cross Currency Interest Rate Swaps) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2019	Sep. 30, 2018
Interest Rate Swaps Contracts | Designated as Hedging Instrument | Fair Value Hedges
Derivative [Line Items]
US$ Notional	$ 600.0	$ 600.0
Average Pay %	LIBOR	LIBOR
Average Receive %	2.60%	2.60%
Years Average Maturity	1 year 1 month 6 days	1 year 7 months 6 days
Cross Currency Interest Rate Swaps | Designated as Hedging Instrument | Net Investment Hedges
Derivative [Line Items]
US$ Notional	$ 263.6	$ 201.7
Average Pay %	4.61%	4.42%
Average Receive %	3.12%	2.97%
Years Average Maturity	3 years 2 months 12 days	3 years 1 month 6 days
Cross Currency Interest Rate Swaps | Designated as Hedging Instrument | Cash Flow Hedges
Derivative [Line Items]
US$ Notional	$ 1,024.0	$ 1,052.7
Average Pay %	5.03%	4.99%
Average Receive %	2.99%	2.89%
Years Average Maturity	2 years 2 months 12 days	2 years 3 months 18 days
Cross Currency Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative [Line Items]
US$ Notional	$ 12.6	$ 80.2
Average Pay %	2.55%	4.88%
Average Receive %	3.72%	3.43%
Years Average Maturity	5 years	3 years 10 months 24 days