Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Deferred tax assets:
Inventory costs	$ 67.8	$ 74.0
Inventory impairments	5.8	9.5
Warranty and construction defect costs	281.2	239.5
Net operating loss carryforwards	42.2	44.1
Tax credit carryforwards	6.8	6.4
Incentive compensation plans	88.0	84.7
Other	7.0	12.7
Total deferred tax assets	498.8	470.9
Valuation allowance	(14.8)	(17.9)
Total deferred tax assets, net of valuation allowance	484.0	453.0
Deferred tax liabilities:
Deferral of profit on home closings	163.6	209.4
Depreciation of fixed assets	46.3	30.5
Deferred Tax Liabilities, Deferral of Income	23.4	18.5
Deferred Tax Liabilities, Undistributed Earnings of Subsidiary	51.7	27.3
Other	11.8	26.2
Total deferred tax liabilities	296.8	311.9
Deferred income taxes, net	187.2	141.1
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	42.2	$ 44.1
Internal Revenue Service (IRS)
Deferred tax assets:
Net operating loss carryforwards	29.3
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	$ 29.3