Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Sep. 30, 2021
Assets [Abstract]
Cash and cash equivalents	$ 1,655.7		$ 1,655.7		$ 3,210.4
Restricted cash	28.6		28.6		26.8
Inventories:
Construction in progress and finished homes	10,803.1		10,803.1		7,739.2
Residential land and lots — developed and under development	8,787.6		8,787.6		7,781.8
Land held for development	124.5		124.5		110.9
Land held for sale	32.7		32.7		25.4
Rental properties	1,951.0		1,951.0		821.8
Total inventories	21,698.9		21,698.9		16,479.1
Mortgage loans held for sale	2,082.5		2,082.5		2,027.3
Deferred income taxes, net	157.4		157.4		155.3
Property and equipment, net	481.3		481.3		392.9
Other assets	2,602.0		2,602.0		1,560.6
Goodwill	163.5		163.5		163.5
Total assets	28,869.9		28,869.9		24,015.9
Liabilities [Abstract]
Accounts payable	1,507.3		1,507.3		1,177.0
Accrued expenses and other liabilities	2,951.9		2,951.9		2,210.3
Notes payable	5,975.2		5,975.2		5,412.4
Total liabilities	10,434.4		10,434.4		8,799.7
Revenues
Total revenues	8,788.1	$ 7,284.6	23,840.6	$ 19,664.9
Inventory and land option charges	10.6	5.6	30.6	17.6
Cost of sales	5,879.3	5,212.6	16,214.9	14,196.0
Selling, General and Administrative Expense	(740.6)	(655.7)	(2,101.6)	(1,863.2)
Loss on extinguishment of debt	0.0	(18.1)	0.0	(18.1)
Gain on sale of assets	0.0	0.0	0.0	(14.0)
Other (income) expense	(15.1)	(17.4)	(39.8)	(28.2)
Income (loss) before income taxes	2,183.3	1,415.6	5,563.9	3,629.8
Depreciation and amortization			60.2	63.6
Cash provided by (used in) operating activities			(562.8)	(34.5)
HomeBuildingMember
Revenues
Home sales	8,336.4	7,040.1	22,492.0	18,909.2
Cost of Goods and Services Sold	5,774.3	5,179.2	15,862.1	14,093.0
Land [Member]
Revenues
Home sales	87.7	55.8	198.7	154.6
Cost of Goods and Services Sold	43.5	27.8	101.5	85.4
Financial Services [Member]
Revenues
Home sales	254.3	188.7	660.8	601.1
Rental
Revenues
Home sales	109.7	0.0	489.1	0.0
Cost of Goods and Services Sold	50.9	0.0	220.7	0.0
Intersegment Eliminations [Member]
Assets [Abstract]
Cash and cash equivalents	42.0		42.0		10.9
Restricted cash	0.0		0.0		0.0
Inventories:
Construction in progress and finished homes	(170.9)		(170.9)		(108.8)
Residential land and lots — developed and under development	(72.4)		(72.4)		(46.9)
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Rental properties	(25.2)		(25.2)		(19.1)
Total inventories	(268.5)		(268.5)		(174.8)
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	12.1		12.1		(3.9)
Property and equipment, net	124.0		124.0		82.6
Other assets	151.8		151.8		(62.0)
Goodwill	29.2		29.2		29.2
Total assets	90.6		90.6		(118.0)
Liabilities [Abstract]
Accounts payable	(77.1)		(77.1)		0.0
Accrued expenses and other liabilities	(164.2)		(164.2)		(168.5)
Notes payable	0.0		0.0		(0.7)
Total liabilities	(241.3)		(241.3)		(169.2)
Revenues
Total revenues	(232.2)	(287.3)	(981.0)	(847.9)
Inventory and land option charges	0.0	0.0	0.0	0.0
Cost of sales	(249.9)	(283.0)	(961.1)	(828.7)
Selling, General and Administrative Expense	(3.2)	(2.3)	(8.8)	(6.7)
Loss on extinguishment of debt		0.0		0.0
Gain on sale of assets				(14.0)
Other (income) expense	9.5	3.7	20.7	19.6
Income (loss) before income taxes	5.0	(10.3)	(49.4)	(31.5)
Depreciation and amortization			9.3	7.5
Cash provided by (used in) operating activities			(0.8)	164.8
Intersegment Eliminations [Member] | HomeBuildingMember
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	(52.5)	(40.0)	(133.9)	(103.3)
Intersegment Eliminations [Member] | Land [Member]
Revenues
Home sales	(232.2)	(264.2)	(981.0)	(793.0)
Cost of Goods and Services Sold	(197.4)	(231.3)	(822.1)	(695.8)
Intersegment Eliminations [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Intersegment Eliminations [Member] | Rental
Revenues
Home sales	0.0	(23.1)	0.0	(54.9)
Cost of Goods and Services Sold	0.0	(11.7)	(5.1)	(29.6)
HomeBuildingMember
Assets [Abstract]
Cash and cash equivalents	1,207.2		1,207.2		2,950.1
Restricted cash	16.2		16.2		8.4
Inventories:
Construction in progress and finished homes	10,974.0		10,974.0		7,848.0
Residential land and lots — developed and under development	6,909.4		6,909.4		6,004.0
Land held for development	20.8		20.8		30.4
Land held for sale	32.7		32.7		25.4
Rental properties	0.0		0.0		0.0
Total inventories	17,936.9		17,936.9		13,907.8
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	145.3		145.3		159.2
Property and equipment, net	346.2		346.2		303.3
Other assets	2,113.5		2,113.5		1,468.7
Goodwill	134.3		134.3		134.3
Total assets	21,899.6		21,899.6		18,931.8
Liabilities [Abstract]
Accounts payable	1,308.4		1,308.4		1,073.7
Accrued expenses and other liabilities	2,530.4		2,530.4		1,941.3
Notes payable	3,688.9		3,688.9		3,214.0
Total liabilities	7,527.7		7,527.7		6,229.0
Revenues
Total revenues	8,347.8	7,047.2	22,534.0	18,949.7
Inventory and land option charges	9.5	4.9	23.2	16.0
Cost of sales	5,843.6	5,226.8	16,046.9	14,241.3
Selling, General and Administrative Expense	(553.2)	(500.7)	(1,558.1)	(1,417.8)
Loss on extinguishment of debt		0.0		0.0
Gain on sale of assets				0.0
Other (income) expense	(3.7)	(4.9)	(11.6)	(8.7)
Income (loss) before income taxes	1,954.7	1,324.6	4,940.6	3,299.3
Depreciation and amortization			47.0	47.6
Cash provided by (used in) operating activities			124.5	450.7
HomeBuildingMember | HomeBuildingMember
Revenues
Home sales	8,336.4	7,040.1	22,492.0	18,909.2
Cost of Goods and Services Sold	5,826.8	5,219.2	15,996.0	14,196.3
HomeBuildingMember | Land [Member]
Revenues
Home sales	11.4	7.1	42.0	40.5
Cost of Goods and Services Sold	7.3	2.7	27.7	29.0
HomeBuildingMember | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
HomeBuildingMember | Rental
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	146.3		146.3		153.6
Restricted cash	0.0		0.0		0.0
Inventories:
Construction in progress and finished homes	0.0		0.0		0.0
Residential land and lots — developed and under development	1,950.6		1,950.6		1,824.7
Land held for development	103.7		103.7		80.5
Land held for sale	0.0		0.0		0.0
Rental properties	0.0		0.0		0.0
Total inventories	2,054.3		2,054.3		1,905.2
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	5.5		5.5		2.9
Other assets	61.7		61.7		40.0
Goodwill	0.0		0.0		0.0
Total assets	2,267.8		2,267.8		2,101.7
Liabilities [Abstract]
Accounts payable	63.8		63.8		47.4
Accrued expenses and other liabilities	350.5		350.5		333.9
Notes payable	705.6		705.6		704.5
Total liabilities	1,119.9		1,119.9		1,085.8
Revenues
Total revenues	308.5	312.9	1,137.7	907.1
Inventory and land option charges	1.0	0.7	7.0	1.6
Cost of sales	234.6	257.1	902.9	753.8
Selling, General and Administrative Expense	(24.1)	(16.9)	(69.9)	(48.7)
Loss on extinguishment of debt		(18.1)		(18.1)
Gain on sale of assets				0.0
Other (income) expense	(2.9)	(0.3)	(4.5)	(1.4)
Income (loss) before income taxes	52.7	21.1	169.4	87.9
Depreciation and amortization			2.0	2.1
Cash provided by (used in) operating activities			(10.2)	(340.6)
Forestar Group [Member] | HomeBuildingMember
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Forestar Group [Member] | Land [Member]
Revenues
Home sales	308.5	312.9	1,137.7	907.1
Cost of Goods and Services Sold	233.6	256.4	895.9	752.2
Forestar Group [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Forestar Group [Member] | Rental
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	126.3		126.3		79.0
Restricted cash	11.5		11.5		18.0
Inventories:
Construction in progress and finished homes	0.0		0.0		0.0
Residential land and lots — developed and under development	0.0		0.0		0.0
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Rental properties	0.0		0.0		0.0
Total inventories	0.0		0.0		0.0
Mortgage loans held for sale	2,082.5		2,082.5		2,027.3
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	4.0		4.0		3.5
Other assets	256.7		256.7		107.6
Goodwill	0.0		0.0		0.0
Total assets	2,481.0		2,481.0		2,235.4
Liabilities [Abstract]
Accounts payable	0.0		0.0		0.0
Accrued expenses and other liabilities	223.1		223.1		88.6
Notes payable	1,405.7		1,405.7		1,494.6
Total liabilities	1,628.8		1,628.8		1,583.2
Revenues
Total revenues	254.3	188.7	660.8	601.1
Inventory and land option charges	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Selling, General and Administrative Expense	(137.3)	(127.0)	(400.6)	(360.4)
Loss on extinguishment of debt		0.0		0.0
Gain on sale of assets				0.0
Other (income) expense	(11.3)	(8.6)	(28.0)	(21.4)
Income (loss) before income taxes	128.3	70.3	288.2	262.1
Depreciation and amortization			1.4	1.3
Cash provided by (used in) operating activities			150.1	96.2
Financial Services [Member] | HomeBuildingMember
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Financial Services [Member] | Land [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Financial Services [Member] | Financial Services [Member]
Revenues
Home sales	254.3	188.7	660.8	601.1
Financial Services [Member] | Rental
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Rental
Assets [Abstract]
Cash and cash equivalents	133.9		133.9		16.8
Restricted cash	0.9		0.9		0.4
Inventories:
Construction in progress and finished homes	0.0		0.0		0.0
Residential land and lots — developed and under development	0.0		0.0		0.0
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Rental properties	1,976.2		1,976.2		840.9
Total inventories	1,976.2		1,976.2		840.9
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	1.6		1.6		0.6
Other assets	18.3		18.3		6.3
Goodwill	0.0		0.0		0.0
Total assets	2,130.9		2,130.9		865.0
Liabilities [Abstract]
Accounts payable	212.2		212.2		55.9
Accrued expenses and other liabilities	12.1		12.1		15.0
Notes payable	175.0		175.0		0.0
Total liabilities	399.3		399.3		70.9
Revenues
Total revenues	109.7	23.1	489.1	54.9
Inventory and land option charges	0.1	0.0	0.4	0.0
Cost of sales	51.0	11.7	226.2	29.6
Selling, General and Administrative Expense	(22.8)	(8.8)	(64.2)	(29.6)
Loss on extinguishment of debt		0.0		0.0
Gain on sale of assets				0.0
Other (income) expense	(6.7)	(7.3)	(16.4)	(16.3)
Income (loss) before income taxes	42.6	9.9	215.1	12.0
Depreciation and amortization			0.5	5.1
Cash provided by (used in) operating activities			(826.4)	(405.6)
Rental | HomeBuildingMember
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Rental | Land [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Rental | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Rental | Rental
Revenues
Home sales	109.7	23.1	489.1	54.9
Cost of Goods and Services Sold	50.9	11.7	225.8	29.6
Northwest | HomeBuildingMember
Inventories:
Total inventories	1,757.1		1,757.1		1,307.5
Revenues
Total revenues	674.0	662.6	1,879.9	1,759.1
Income (loss) before income taxes	161.0	146.7	421.0	327.4
Southwest [Member] | HomeBuildingMember
Inventories:
Total inventories	2,985.4		2,985.4		2,445.6
Revenues
Total revenues	1,289.2	1,021.7	3,335.9	2,773.3
Income (loss) before income taxes	288.3	167.5	674.2	413.2
South Central [Member] | HomeBuildingMember
Inventories:
Total inventories	4,408.8		4,408.8		3,479.3
Revenues
Total revenues	2,080.4	1,547.5	5,613.1	4,295.7
Income (loss) before income taxes	527.2	299.0	1,297.5	790.9
Southeast [Member] | HomeBuildingMember
Inventories:
Total inventories	4,066.7		4,066.7		3,178.6
Revenues
Total revenues	2,118.9	1,961.3	5,876.4	5,146.9
Income (loss) before income taxes	532.4	396.1	1,430.1	960.5
East [Member] | HomeBuildingMember
Inventories:
Total inventories	2,638.0		2,638.0		1,919.6
Revenues
Total revenues	1,385.8	1,228.0	3,680.1	3,306.6
Income (loss) before income taxes	309.9	222.8	778.1	578.7
North | HomeBuildingMember
Inventories:
Total inventories	1,870.9		1,870.9		1,368.9
Revenues
Total revenues	799.5	626.1	2,148.6	1,668.1
Income (loss) before income taxes	135.9	$ 92.5	339.7	$ 228.6
Corporate, Non-Segment [Member] | HomeBuildingMember
Inventories:
Total inventories	$ 210.0		$ 210.0		$ 208.3