Capitalized Interest (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Capitalized Interest Costs [Line Items]
Interest Costs Incurred	$ 10.6	$ 2.9	$ 20.5	$ 5.7
Real Estate Inventory, Capitalized Interest Costs	37.9	7.7	37.9	7.7	$ 29.8	$ 23.7	$ 5.3	$ 3.2
Real Estate Inventory, Capitalized Interest Costs, Cost of Sales	$ (2.5)	$ (0.5)	$ (6.3)	$ (1.2)