Commitments and Contingent Liabilities - Narrative (Details) $ in Millions	12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Loss Contingencies [Line Items]
Reasonably possible costs from environmental remediation costs and indemnification liabilities	$ 108.0
Reasonably possible costs from environmental remediation costs and indemnification liabilities, net of related receivables	$ 90.5
Number of Superfund sites designated potentially responsible party at	13
Indemnification liability	$ 93.0	$ 100.8
Related receivable	14.1	17.8
Rental expense	120.3	115.1	$ 115.5
Indemnification Agreement
Loss Contingencies [Line Items]
Indemnification liability	9.5	12.0
Related receivable	4.4	6.2
Substantially Indemnified by ExxonMobil Corporation | Indemnification Agreement
Loss Contingencies [Line Items]
Indemnification liability	4.7	6.6
Related receivable	$ 4.5	$ 6.2