Business Segment Information - Narrative (Details) $ in Millions	3 Months Ended
	Dec. 31, 2020 USD ($) segment	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Assets	$ 8,171.0	$ 7,264.7	$ 7,264.7
Corporate
Segment Reporting Information [Line Items]
Assets	3,147.4		$ 2,692.0
Corporate | Shared Net Property
Segment Reporting Information [Line Items]
Assets	$ 253.3	$ 247.3