Commitments and Contingent Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Loss Contingencies [Line Items]
Environmental remediation costs	$ 73.9	$ 61.4
Conditional asset retirement obligations	20.6	20.2
Indemnification liabilities	111.5	114.2
Recorded probable expected recoveries	(22.5)	(33.2)
Net recorded liabilities	89.0	81.0
Indemnification Agreement [Member]
Loss Contingencies [Line Items]
Other current liabilities	4.9	3.2
Other liabilities	12.1	29.4
Indemnification liabilities	17.0	32.6
Receivables	(3.5)	(2.1)
Other assets	(7.3)	(22.6)
Recorded probable expected recoveries	(10.8)	(24.7)
Net recorded liabilities	6.2	7.9
Substantially Indemnified by ExxonMobil Corporation [Member] | Indemnification Agreement [Member]
Loss Contingencies [Line Items]
Indemnification liabilities	11.0	26.0
Recorded probable expected recoveries	$ (10.8)	$ (24.7)