Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Deferred tax assets:
Inventory costs	$ 39.6	$ 87.3
Inventory impairments	13.4	8.7
Warranty and construction defect costs	356.2	316.2
Net operating loss carryforwards	36.5	38.9
Tax credit carryforwards	22.1	6.9
Incentive compensation plans	105.0	93.7
Other	21.2	14.0
Total deferred tax assets	594.0	565.7
Valuation allowance	(14.6)	(14.9)
Total deferred tax assets, net of valuation allowance	579.4	550.8
Deferred tax liabilities:
Deferral of profit on home closings	327.6	226.4
Depreciation of fixed assets	76.7	44.8
Deferred Tax Liabilities, Deferral of Income	31.9	29.3
Deferred Tax Liabilities, Undistributed Earnings of Subsidiary	90.7	77.4
Other	8.0	5.4
Total deferred tax liabilities	534.9	383.3
Deferred income taxes, net	44.5	167.5
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	36.5	$ 38.9
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	16.5
State and Local Jurisdiction [Member] \| Home Building
Deferred tax assets:
Net operating loss carryforwards	10.4
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	10.4
State and Local Jurisdiction [Member] \| Home Building \| NOL Carryforwards to Expire in One to Ten Years [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	5.5
State and Local Jurisdiction [Member] \| Home Building \| NOL Carryforward to Expire in Eleven to Twenty Years [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	4.9
State and Local Jurisdiction [Member] \| Forestar Group [Member]
Deferred tax assets:
Net operating loss carryforwards	0.7
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	0.7
Internal Revenue Service (IRS)
Deferred tax assets:
Net operating loss carryforwards	25.4
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	$ 25.4

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