Inventory (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Inventory [Line Items]
Carrying value of communities with impairment indicators	$ 52.1		$ 52.1
Impairment charges	0.0	$ 6.8	1.7	$ 18.6
Write-offs (recoveries) of earnest money deposits and pre-acquisition costs	$ 5.0	$ 12.4	$ 16.1	$ 22.4