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Notes Payable - Principal Amounts (Details) - USD ($) $ in Millions	Sep. 30, 2021	Oct. 31, 2020	Sep. 30, 2020
Debt Instrument [Line Items]
Notes payable, carrying value	$ 5,412.4		$ 4,283.3
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	1,900.0
Long-term Debt, Maturities, Repayments of Principal in Year Two	700.3
Long-term Debt, Maturities, Repayments of Principal in Year Three	13.7
Long-term Debt, Maturities, Repayments of Principal in Year Four	500.4
Long-term Debt, Maturities, Repayments of Principal in Year Five	900.4
Long-Term Debt, Maturity, after Year Five	1,400.0
Intersegment Eliminations [Member]
Debt Instrument [Line Items]
Notes payable, carrying value	(0.7)		(4.8)
Home Building Consolidated
Debt Instrument [Line Items]
Line of credit, amount outstanding	0.0		0.0
Line of Credit, Current	0.0		0.0
Notes payable, carrying value	3,213.3		2,509.6
Unamortized Debt Issuance Expense	16.5		10.7
Home Building Consolidated \| Secured Debt [Member]
Debt Instrument [Line Items]
Notes payable, carrying value	$ 82.2		71.1
Home Building Consolidated \| Senior Note Member Thirty-One [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate	2.55%
Notes payable, carrying value	$ 0.0		399.8
Home Building Consolidated \| Senior Note Member Twenty Five [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate	4.375%
Notes payable, carrying value	$ 349.6		349.2
Home Building Consolidated \| Senior Note Twenty Seven [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate	4.75%
Notes payable, carrying value	$ 299.5		299.2
Home Building Consolidated \| SeniorNoteTwentyEight [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate	5.75%
Notes payable, carrying value	$ 399.1		398.7
Home Building Consolidated \| SeniorNoteFortyTwo [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate	2.50%	1.40%
Notes payable, carrying value	$ 497.3		496.5
Home Building Consolidated \| SeniorNoteFortyFour
Debt Instrument [Line Items]
Senior notes, stated interest rate	2.60%
Notes payable, carrying value	$ 496.2		495.1
Home Building Consolidated \| SeniorNoteFortySIx
Debt Instrument [Line Items]
Senior notes, stated interest rate	1.30%
Notes payable, carrying value	$ 594.5		0.0
Home Building Consolidated \| Senior Note Forty Five [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate	1.40%
Notes payable, carrying value	$ 494.9		0.0
Forestar Consolidated [Member]
Debt Instrument [Line Items]
Line of credit, amount outstanding	0.0		0.0
Notes payable, carrying value	704.5		641.1
Unamortized Debt Issuance Expense	8.0		8.9
Forestar Consolidated [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Notes payable, carrying value	$ 12.5		0.0
Forestar Consolidated [Member] \| Senior Note Member Forty One [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate	8.00%
Notes payable, carrying value	$ 0.0		345.2
Forestar Consolidated [Member] \| Senior Note Forty Seven
Debt Instrument [Line Items]
Senior notes, stated interest rate	3.85%
Notes payable, carrying value	$ 395.5		0.0
Forestar Consolidated [Member] \| Senior Note Member Forty Three
Debt Instrument [Line Items]
Senior notes, stated interest rate	5.00%
Notes payable, carrying value	$ 296.5		295.9
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable, carrying value	$ 1,494.6		$ 1,132.6