Notes Payable - Principal Amounts (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024
Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	$ 7,200.0	$ 5,900.0
Notes payable	7,248.7	5,917.7
Fair Value, Inputs, Level 2 [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	4,300.0	3,400.0
Fair Value, Inputs, Level 3 [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	2,900.0	2,500.0
Home Building Consolidated
Debt Instrument [Line Items]
Revolving credit facility	0.0	0.0
Notes payable	3,651.7	2,926.8
Unamortized Debt Issuance Expense	20.0	11.7
Home Building Consolidated \| Unsecured Debt
Debt Instrument [Line Items]
Notes payable	179.2	146.2
Rental
Debt Instrument [Line Items]
Revolving credit facility	1,020.0	745.0
Notes payable	1,020.0	750.7
Rental \| Unsecured Debt
Debt Instrument [Line Items]
Notes payable	0.0	5.7
Forestar Group [Member]
Debt Instrument [Line Items]
Revolving credit facility	0.0	0.0
Notes payable	872.8	706.4
Forestar Group [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Notes payable	9.9	9.9
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable	1,704.2	1,533.8
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	872.8	706.4
Unamortized Debt Issuance Expense	7.6	3.5
2.5% Senior Notes due 2024 [Member] \| Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 0.0	500.0
Debt Instrument, Interest Rate, Stated Percentage	2.50%
2.6% Senior Notes due 2025 [Member] \| Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 499.7	499.0
Debt Instrument, Interest Rate, Stated Percentage	2.60%
1.3% Senior Notes due 2026 [Member] \| Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 598.5	597.7
Debt Instrument, Interest Rate, Stated Percentage	1.30%
1.4% Senior Notes due 2027 [Member] \| Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 498.0	497.4
Debt Instrument, Interest Rate, Stated Percentage	1.40%
4.85% Senior Notes due 2030 \| Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 495.4	0.0
Debt Instrument, Interest Rate, Stated Percentage	4.85%
5.0% Senior Notes due 2034 [Member] \| Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 687.5	686.5
Debt Instrument, Interest Rate, Stated Percentage	5.00%
5.5% Senior Notes due 2035 [Member] \| Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 693.4	0.0
Debt Instrument, Interest Rate, Stated Percentage	5.50%
3.85% Senior Notes due 2026 [Member] \| Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	$ 70.4	398.4
Debt Instrument, Interest Rate, Stated Percentage	3.85%
5.0% Senior Notes due 2028 [Member] \| Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	$ 298.5	298.1
Debt Instrument, Interest Rate, Stated Percentage	5.00%
6.5% Senior Notes due 2033 [Member] \| Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	$ 494.0	0.0
Debt Instrument, Interest Rate, Stated Percentage	6.50%
Commitments to Extend Credit [Member] \| Financial Services [Member]
Debt Instrument [Line Items]
Notes payable	$ 1,252.5	1,229.3
Warehouse Agreement Borrowings [Member] \| Financial Services [Member]
Debt Instrument [Line Items]
Notes payable	$ 451.7	$ 304.5

Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Fair value portion of notes payable.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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