Commitments and Contingent Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Loss Contingencies [Line Items]
Environmental remediation costs	$ 67.6	$ 73.9
Conditional asset retirement obligations	21.2	20.6
Indemnification liabilities	100.8	111.5
Recorded probable expected recoveries	(17.8)	(22.5)
Net recorded liabilities	83.0	89.0
Indemnification Agreement [Member]
Loss Contingencies [Line Items]
Other current liabilities	2.5	4.9
Other liabilities	9.5	12.1
Indemnification liabilities	12.0	17.0
Receivables	(1.6)	(3.5)
Other assets	(4.6)	(7.3)
Recorded probable expected recoveries	(6.2)	(10.8)
Net recorded liabilities	5.8	6.2
Substantially Indemnified by ExxonMobil Corporation [Member] | Indemnification Agreement [Member]
Loss Contingencies [Line Items]
Indemnification liabilities	6.6	11.0
Recorded probable expected recoveries	$ (6.2)	$ (10.8)