Inventory (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Inventory [Line Items]
Carrying value of communities with impairment indicators	$ 47.6		$ 47.6
Impairment charges	1.7	$ 7.7	1.7	$ 11.9
Write-offs (recoveries) of earnest money deposits and pre-acquisition costs	$ 7.2	$ 6.1	$ 11.1	$ 9.9