Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Inventory costs	$ 37.6	$ 39.8
Inventory impairments	21.5	27.9
Warranty and construction defect costs	156.7	135.1
Net operating loss carryforwards	17.5	31.5
Tax credit carryforwards	2.1	3.5
Incentive compensation plans	68.3	65.1
Other	15.6	7.2
Total deferred tax assets	319.3	310.1
Valuation allowance	(7.5)	(18.7)
Total deferred tax assets, net of valuation allowance	311.8	291.4
Deferred tax liabilities:
Deferral of profit on home sales	131.9	95.4
Deferred Tax Liabilities, Property, Plant and Equipment	19.0	14.2
Other	16.0	18.7
Total deferred tax liabilities	166.9	128.3
Deferred income taxes, net	$ 144.9	$ 163.1