Income Taxes - Deferred Tax Liabilities (Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Excess book basis over tax basis of property, plant and equipment	$ 804.5	$ 807.8
Investment in AmeriGas Partners	0.0	219.2
Utility regulatory assets	107.6	86.7
Intangible assets and goodwill	70.5	67.6
Derivative instruments	0.0	30.4
Other	8.3	10.6
Gross deferred tax liabilities	990.9	1,222.3
Investment in AmeriGas Partners	(303.5)	0.0
Pension plan liabilities	(49.9)	(20.0)
Employee-related benefits	(43.8)	(43.6)
Operating loss carryforwards	(21.2)	(26.2)
Foreign tax credit carryforwards	(81.4)	(106.1)
Utility regulatory liabilities	(94.3)	(118.6)
Derivative instruments	(15.3)	0.0
Utility environmental liabilities	(15.3)	(14.7)
Other	(33.4)	(29.0)
Gross deferred tax assets	(658.1)	(358.2)
Deferred tax assets valuation allowance	90.7	116.8
Net deferred tax liabilities	$ 423.5	$ 980.9