Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Deferred tax assets:
Inventory costs	$ 87.3	$ 67.8
Inventory impairments	8.7	5.8
Warranty and construction defect costs	316.2	281.2
Net operating loss carryforwards	38.9	42.2
Tax credit carryforwards	6.9	6.8
Incentive compensation plans	93.7	88.0
Other	14.0	7.0
Total deferred tax assets	565.7	498.8
Valuation allowance	(14.9)	(14.8)
Total deferred tax assets, net of valuation allowance	550.8	484.0
Deferred tax liabilities:
Deferral of profit on home closings	226.4	163.6
Depreciation of fixed assets	44.8	46.3
Deferred Tax Liabilities, Deferral of Income	29.3	23.4
Deferred Tax Liabilities, Undistributed Earnings of Subsidiary	77.4	51.7
Other	5.4	11.8
Total deferred tax liabilities	383.3	296.8
Deferred income taxes, net	167.5	187.2
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	38.9	$ 42.2
Internal Revenue Service (IRS)
Deferred tax assets:
Net operating loss carryforwards	27.1
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	$ 27.1