Notes Payable (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2024	Sep. 30, 2023
Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	$ 5,800.0	$ 4,800.0
Letters of Credit Outstanding, Amount	241.7
Notes payable	5,937.9	5,094.5
Fair Value, Inputs, Level 2 [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	2,600.0	2,500.0
Fair Value, Inputs, Level 3 [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	3,200.0	2,300.0
Forestar Group [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	4.2	5.0
Notes payable	705.7	695.0
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable	1,884.0	1,669.6
Forestar Group [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	410.0
Line of Credit Facility, Maximum Borrowing Capacity	$ 600.0
Letters of credit, sublimit borrowing capacity, as a percentage	50.00%
Revolving credit facility	$ 0.0	0.0
Letters of Credit Outstanding, Amount	28.0
Line of Credit Facility, Remaining Borrowing Capacity	382.0
Authorized Repurchase Of Debt Securities	30.0
Debt Repurchase Authorization Remaining	30.0
Letter of Credit, Maximum Borrowing Capacity (in dollars)	100.0
Notes payable	705.7	695.0
Home Building Consolidated
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	6.7	8.4
Line of Credit Facility, Current Borrowing Capacity	2,190.0
Line of Credit Facility, Maximum Borrowing Capacity	$ 3,000.0
Letters of credit, sublimit borrowing capacity, as a percentage	100.00%
Revolving credit facility	$ 0.0	0.0
Letters of Credit Outstanding, Amount	213.7
Line of Credit Facility, Remaining Borrowing Capacity	1,980.0
Authorized Repurchase Of Debt Securities	500.0
Debt Repurchase Authorization Remaining	500.0
Notes payable	2,363.2	2,329.9
Rental
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	1,050.0
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,000.0
Letters of credit, sublimit borrowing capacity, as a percentage	50.00%
Revolving credit facility	$ 985.0	400.0
Letters of Credit Outstanding, Amount	0.0
Line of Credit Facility, Remaining Borrowing Capacity	65.0
Letter of Credit, Maximum Borrowing Capacity (in dollars)	100.0
Notes payable	$ 985.0	400.0
Line of Credit Facility, Interest Rate at Period End	7.40%
Proceeds from Lines of Credit	$ 985.0
Repayments of Lines of Credit	$ 400.0
SeniorNoteFortyTwo [Member] | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.50%
Notes payable	$ 499.5	499.0
SeniorNoteFortyFour | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.60%
Notes payable	$ 498.5	498.0
Senior Note Forty Six | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.30%
Notes payable	$ 597.1	596.6
Senior Note Forty Five | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.40%
Notes payable	$ 496.9	496.5
Senior Note Member Forty Six | Forestar Group [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.85%
Notes payable	$ 397.9	397.4
Senior Note Member Forty Three | Forestar Group [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.00%
Notes payable	$ 297.9	297.6
Secured Debt [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	9.9	0.0
Secured Debt [Member] | Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	271.2	239.8
Commitments to Extend Credit | Financial Services [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	1,600.0
Line of Credit Facility, Maximum Borrowing Capacity	2,000.0
Deposit Liabilities, Collateral Issued, Financial Instruments	2,170.0
Participating Mortgage Loans, Mortgage Obligations, Amount	$ 2,120.0
Assets Sold under Agreements to Repurchase, Interest Rate	7.00%
Notes payable	$ 1,390.3	1,373.3
Warehouse Agreement Borrowings | Financial Services [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	500.0
Deposit Liabilities, Collateral Issued, Financial Instruments	540.4
Participating Mortgage Loans, Mortgage Obligations, Amount	$ 511.0
Assets Sold under Agreements to Repurchase, Interest Rate	6.50%
Notes payable	$ 493.7	$ 296.3