Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Business Information [Line Items]
Cash and cash equivalents	$ 3,873.6	$ 2,540.5
Advertising expense	64.7	41.7	$ 36.1
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	1.4	2.9
Variable Interest Entity, Consolidated, Notes Payable	$ 118.8	64.3
Forestar Group [Member]
Business Information [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	63.00%
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	37.00%
Subsidiaries [Member]
Business Information [Line Items]
Cash and cash equivalents	$ 70.2	64.4
Variable Interest Entity, Not Primary Beneficiary [Member]
Business Information [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	1,600.0	1,500.0
Accounts Receivable [Member]
Business Information [Line Items]
Cash and cash equivalents	$ 301.8	$ 494.8