Income Taxes - Deferred Tax Liabilities (Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Excess book basis over tax basis of property, plant and equipment	$ 975.8	$ 873.9
Investment in AmeriGas Partners	326.8	323.2
Intangible assets and goodwill	98.2	87.1
Utility regulatory assets	132.2	148.3
Other	11.7	11.9
Gross deferred tax liabilities	1,544.7	1,444.4
Pension plan liabilities	(57.7)	(79.7)
Employee-related benefits	(65.4)	(63.1)
Operating loss carryforwards	(30.9)	(31.5)
Foreign tax credit carryforwards	(106.1)	(105.1)
Utility regulatory liabilities	(9.3)	(13.9)
Derivative instruments	(1.7)	(14.7)
Utility environmental liabilities	(22.2)	(22.8)
Other	(27.8)	(28.3)
Gross deferred tax assets	(321.1)	(359.1)
Deferred tax assets valuation allowance	107.1	114.3
Net deferred tax liabilities	$ 1,330.7	$ 1,199.6