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, 2018	Sep. 30, 2017
Interest Rate Swaps Contracts | Designated as Hedging Instrument | Fair Value Hedges
Derivative [Line Items]
US$ Notional	$ 400.0	$ 600.0
Average Pay %	LIBOR	LIBOR
Average Receive %	2.53%	2.28%
Years Average Maturity	1 year 4 months 24 days	1 year 3 months 18 days
Cross Currency Interest Rate Swaps | Designated as Hedging Instrument | Net Investment Hedges
Derivative [Line Items]
US$ Notional	$ 688.7	$ 539.7
Average Pay %	3.73%	3.27%
Average Receive %	2.76%	2.59%
Years Average Maturity	2 years 3 months 18 days	1 year 10 months 24 days
Cross Currency Interest Rate Swaps | Designated as Hedging Instrument | Cash Flow Hedges
Derivative [Line Items]
US$ Notional	$ 1,014.6	$ 1,095.7
Average Pay %	5.01%	4.96%
Average Receive %	2.75%	2.78%
Years Average Maturity	2 years 4 months 24 days	2 years 4 months 24 days
Cross Currency Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative [Line Items]
US$ Notional	$ 39.5	$ 41.6
Average Pay %	3.16%	3.28%
Average Receive %	2.88%	2.32%
Years Average Maturity	1 year 6 months	1 year 8 months 12 days