Notes Payable (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2023	Mar. 31, 2023	Sep. 30, 2022	Sep. 30, 2020	Sep. 30, 2019
Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	$ 5,700.0	$ 5,700.0	$ 5,700.0
Letters of Credit Outstanding, Amount	245.2	245.2
Notes payable	5,966.9	5,966.9	6,066.9
Fair Value, Inputs, Level 2 [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	2,900.0	2,900.0	3,100.0
Fair Value, Inputs, Level 3 [Member]
Debt Instrument [Line Items]
Notes payable, Fair value	2,800.0	2,800.0	2,600.0
Intersegment Eliminations [Member]
Debt Instrument [Line Items]
Notes payable	0.0	0.0	0.0
Forestar Group [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	5.7	5.7	6.5
Notes payable	706.8	706.8	706.0
Financial Services [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	2,000.0	2,000.0
Line of Credit Facility, Maximum Borrowing Capacity	2,300.0	2,300.0
Deposit Liabilities, Collateral Issued, Financial Instruments	2,140.0	2,140.0
Participating Mortgage Loans, Mortgage Obligations, Amount	2,100.0	$ 2,100.0
Advance Pay Downs on Mortgage Repurchase Facility	$ 541.0
Assets Sold under Agreements to Repurchase, Interest Rate	6.40%	6.40%
Notes payable	$ 1,554.9	$ 1,554.9	1,618.3
Forestar Group [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	410.0	410.0
Line of Credit Facility, Maximum Borrowing Capacity	$ 600.0	$ 600.0
Letters of credit, sublimit borrowing capacity, as a percentage	50.00%	50.00%
Revolving credit facility	$ 0.0	$ 0.0	0.0
Letters of Credit Outstanding, Amount	42.6	42.6
Line of Credit Facility, Remaining Borrowing Capacity	367.4	367.4
Authorized Repurchase Of Debt Securities				$ 30.0
Debt Repurchase Authorization Remaining	30.0	30.0
Letter of Credit, Maximum Borrowing Capacity (in dollars)	100.0	100.0
Notes payable	706.8	706.8	706.0
Home Building Consolidated
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	10.2	10.2	12.2
Line of Credit Facility, Current Borrowing Capacity	2,190.0	2,190.0
Line of Credit Facility, Maximum Borrowing Capacity	$ 3,000.0	$ 3,000.0
Letters of credit, sublimit borrowing capacity, as a percentage	100.00%	100.00%
Revolving credit facility	$ 0.0	$ 0.0	0.0
Letters of Credit Outstanding, Amount	202.6	202.6
Line of Credit Facility, Remaining Borrowing Capacity	1,990.0	1,990.0
Authorized Repurchase Of Debt Securities					$ 500.0
Debt Repurchase Authorization Remaining	500.0	500.0
Notes payable	2,680.2	2,680.2	2,942.6
Rental
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	1,025.0	1,025.0
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,250.0	$ 1,250.0
Letters of credit, sublimit borrowing capacity, as a percentage	50.00%	50.00%
Revolving credit facility	$ 1,025.0	$ 1,025.0	800.0
Letter of Credit, Maximum Borrowing Capacity (in dollars)	100.0	100.0
Notes payable	$ 1,025.0	1,025.0	800.0
Proceeds from Lines of Credit		575.0
Repayments of Lines of Credit		$ 350.0
Line of Credit Facility, Interest Rate at Period End	7.20%	7.20%
4.75% senior notes due 2023 [Member] | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.75%	4.75%
Maturities of Senior Debt	$ 300.0
Notes payable	$ 0.0	$ 0.0	299.9
5.75% senior notes due 2023 [Member] | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.75%	5.75%
Notes payable	$ 399.8	$ 399.8	399.6
SeniorNoteFortyTwo [Member] | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.50%	2.50%
Notes payable	$ 498.6	$ 498.6	498.2
SeniorNoteFortyFour | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.60%	2.60%
Notes payable	$ 497.6	$ 497.6	497.1
Senior Note Forty Six | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.30%	1.30%
Notes payable	$ 596.0	$ 596.0	595.5
Senior Note Forty Five | Home Building Consolidated
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.40%	1.40%
Notes payable	$ 496.1	$ 496.1	495.7
Senior Note Member Forty Six | Forestar Group [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.85%	3.85%
Notes payable	$ 397.0	$ 397.0	396.5
Senior Note Member Forty Three | Forestar Group [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.00%	5.00%
Notes payable	$ 297.3	$ 297.3	297.0
Secured Debt [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	12.5	12.5	12.5
Secured Debt [Member] | Home Building Consolidated
Debt Instrument [Line Items]
Notes payable	$ 192.1	$ 192.1	$ 156.6