Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Assets [Abstract]
Cash and cash equivalents	$ 3,063.1		$ 3,063.1		$ 3,873.6
Restricted cash	30.3		30.3		26.5
Inventories:
Construction in progress and finished homes	9,708.5		9,708.5		9,001.4
Residential land and lots — developed and under development	11,889.9		11,889.9		10,621.9
Land held for development	155.2		155.2		50.0
Land held for sale	8.8		8.8		8.7
Rental properties	3,077.6		3,077.6		2,691.3
Total inventories	24,840.0		24,840.0		22,373.3
Mortgage loans held for sale	2,672.4		2,672.4		2,519.9
Deferred income taxes, net	166.5		166.5		187.2
Property and equipment, net	479.9		479.9		445.4
Other assets	2,982.5		2,982.5		2,993.0
Goodwill	163.5		163.5		163.5
Total assets	34,398.2		34,398.2		32,582.4
Liabilities [Abstract]
Accounts payable	1,386.5		1,386.5		1,246.2
Accrued expenses and other liabilities	2,777.4		2,777.4		3,103.8
Notes payable	5,937.9		5,937.9		5,094.5
Total liabilities	10,101.8		10,101.8		9,444.5
Revenues
Total revenues	9,107.2	$ 7,972.9	16,833.1	$ 15,230.6
Inventory and land option charges	13.6	23.8	19.6	51.4
Cost of sales	6,774.3	5,996.2	12,494.0	11,287.5
Selling, General and Administrative Expense	(880.6)	(773.6)	(1,715.6)	(1,510.5)
Other (income) expense	(76.2)	(42.2)	(152.5)	(79.9)
Income (loss) before income taxes	1,528.5	1,245.3	2,776.0	2,512.5
Depreciation and amortization			41.1	46.5
Net cash (used in) provided by operating activities			(470.1)	1,473.6
HomeBuildingMember
Revenues
Home sales	8,466.7	7,449.7	15,743.1	14,158.9
Cost of Goods and Services Sold	6,437.3	5,781.2	11,991.1	10,838.8
Land [Member]
Revenues
Home sales	43.6	82.7	105.3	166.7
Cost of Goods and Services Sold	20.6	34.4	44.0	58.3
Rental
Revenues
Home sales	371.3	224.1	566.5	551.6
Cost of Goods and Services Sold	302.8	156.8	439.3	339.0
Financial Services [Member]
Revenues
Home sales	225.6	216.4	418.2	353.4
HomeBuildingMember
Assets [Abstract]
Cash and cash equivalents	2,192.6		2,192.6		2,920.2
Restricted cash	6.7		6.7		6.5
Inventories:
Construction in progress and finished homes	9,847.5		9,847.5		9,134.3
Residential land and lots — developed and under development	10,053.1		10,053.1		8,992.3
Land held for development	20.4		20.4		20.5
Land held for sale	8.8		8.8		8.7
Rental properties	0.0		0.0		0.0
Total inventories	19,929.8		19,929.8		18,155.8
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	209.1		209.1		229.8
Property and equipment, net	449.8		449.8		415.0
Other assets	2,630.1		2,630.1		2,838.5
Goodwill	134.3		134.3		134.3
Total assets	25,552.4		25,552.4		24,700.1
Liabilities [Abstract]
Accounts payable	1,120.0		1,120.0		1,033.7
Accrued expenses and other liabilities	2,434.5		2,434.5		2,585.5
Notes payable	2,363.2		2,363.2		2,329.9
Total liabilities	5,917.7		5,917.7		5,949.1
Revenues
Total revenues	8,473.6	7,469.6	15,770.3	14,213.6
Inventory and land option charges	13.1	14.2	18.6	38.4
Cost of sales	6,522.9	5,870.6	12,149.5	11,006.4
Selling, General and Administrative Expense	(614.1)	(545.6)	(1,217.5)	(1,072.6)
Other (income) expense	(21.0)	(14.5)	(50.4)	(27.7)
Income (loss) before income taxes	1,357.6	1,067.9	2,453.7	2,162.3
Depreciation and amortization			37.2	30.7
Net cash (used in) provided by operating activities			408.3	1,456.4
HomeBuildingMember | HomeBuildingMember
Revenues
Home sales	8,466.7	7,449.7	15,743.1	14,158.9
Cost of Goods and Services Sold	6,505.6	5,843.0	12,113.6	10,949.7
HomeBuildingMember | Land [Member]
Revenues
Home sales	6.9	19.9	27.2	54.7
Cost of Goods and Services Sold	4.2	13.4	17.3	18.3
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
HomeBuildingMember | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Rental
Assets [Abstract]
Cash and cash equivalents	117.0		117.0		136.1
Restricted cash	2.5		2.5		3.3
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	3,092.8		3,092.8		2,708.4
Total inventories	3,092.8		3,092.8		2,708.4
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	(19.9)		(19.9)		(19.9)
Property and equipment, net	2.0		2.0		2.4
Other assets	33.6		33.6		29.8
Goodwill	0.0		0.0		0.0
Total assets	3,228.0		3,228.0		2,860.1
Liabilities [Abstract]
Accounts payable	387.1		387.1		698.6
Accrued expenses and other liabilities	36.4		36.4		43.2
Notes payable	985.0		985.0		400.0
Total liabilities	1,408.5		1,408.5		1,141.8
Revenues
Total revenues	371.3	224.1	566.5	551.6
Inventory and land option charges	0.3	0.4	0.6	1.4
Cost of sales	303.1	158.0	444.7	342.8
Selling, General and Administrative Expense	(61.4)	(53.5)	(108.8)	(101.0)
Other (income) expense	(26.5)	(22.0)	(51.6)	(37.1)
Income (loss) before income taxes	33.3	34.6	64.6	144.9
Depreciation and amortization			1.3	1.0
Net cash (used in) provided by operating activities			(653.9)	(263.3)
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 | Rental
Revenues
Home sales	371.3	224.1	566.5	551.6
Cost of Goods and Services Sold	302.8	157.6	444.1	341.4
Rental | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	416.2		416.2		616.0
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	1,981.1		1,981.1		1,760.8
Land held for development	134.8		134.8		29.5
Land held for sale	0.0		0.0		0.0
Rental properties	0.0		0.0		0.0
Total inventories	2,115.9		2,115.9		1,790.3
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	6.1		6.1		5.9
Other assets	60.5		60.5		58.5
Goodwill	0.0		0.0		0.0
Total assets	2,598.7		2,598.7		2,470.7
Liabilities [Abstract]
Accounts payable	61.7		61.7		68.4
Accrued expenses and other liabilities	358.3		358.3		337.4
Notes payable	705.7		705.7		695.0
Total liabilities	1,125.7		1,125.7		1,100.8
Revenues
Total revenues	333.8	301.5	639.7	518.2
Inventory and land option charges	0.2	20.3	0.4	22.7
Cost of sales	250.7	245.6	483.8	414.8
Selling, General and Administrative Expense	(29.2)	(22.0)	(57.2)	(44.9)
Other (income) expense	(5.0)	(2.0)	(11.4)	(5.3)
Income (loss) before income taxes	58.9	35.9	110.1	63.8
Depreciation and amortization			1.4	1.5
Net cash (used in) provided by operating activities			(216.0)	21.3
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	333.8	301.5	639.7	518.2
Cost of Goods and Services Sold	250.5	225.3	483.4	392.1
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
Forestar Group [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	315.2		315.2		189.1
Restricted cash	21.1		21.1		16.7
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	2,672.4		2,672.4		2,519.9
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	3.9		3.9		4.1
Other assets	200.7		200.7		250.3
Goodwill	0.0		0.0		0.0
Total assets	3,213.3		3,213.3		2,980.1
Liabilities [Abstract]
Accounts payable	0.1		0.1		0.1
Accrued expenses and other liabilities	227.8		227.8		280.4
Notes payable	1,884.0		1,884.0		1,669.6
Total liabilities	2,111.9		2,111.9		1,950.1
Revenues
Total revenues	225.6	216.4	418.2	353.4
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	(171.2)	(146.9)	(322.7)	(281.0)
Other (income) expense	(23.6)	(16.1)	(48.5)	(31.4)
Income (loss) before income taxes	78.0	85.6	144.0	103.8
Depreciation and amortization			0.9	1.1
Net cash (used in) provided by operating activities			(40.7)	232.2
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] | 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] | Financial Services [Member]
Revenues
Home sales	225.6	216.4	418.2	353.4
Eliminations and Other
Assets [Abstract]
Cash and cash equivalents	22.1		22.1		12.2
Restricted cash	0.0		0.0		0.0
Inventories:
Construction in progress and finished homes	(139.0)		(139.0)		(132.9)
Residential land and lots — developed and under development	(144.3)		(144.3)		(131.2)
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Rental properties	(15.2)		(15.2)		(17.1)
Total inventories	(298.5)		(298.5)		(281.2)
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	(22.7)		(22.7)		(22.7)
Property and equipment, net	18.1		18.1		18.0
Other assets	57.6		57.6		(184.1)
Goodwill	29.2		29.2		29.2
Total assets	(194.2)		(194.2)		(428.6)
Liabilities [Abstract]
Accounts payable	(182.4)		(182.4)		(554.6)
Accrued expenses and other liabilities	(279.6)		(279.6)		(142.7)
Notes payable	0.0		0.0		0.0
Total liabilities	(462.0)		(462.0)		(697.3)
Revenues
Total revenues	(297.1)	(238.7)	(561.6)	(406.2)
Inventory and land option charges	0.0	(11.1)	0.0	(11.1)
Cost of sales	(302.4)	(278.0)	(584.0)	(476.5)
Selling, General and Administrative Expense	(4.7)	(5.6)	(9.4)	(11.0)
Other (income) expense	(0.1)	12.4	9.4	21.6
Income (loss) before income taxes	0.7	21.3	3.6	37.7
Depreciation and amortization			0.3	12.2
Net cash (used in) provided by operating activities			32.2	27.0
Eliminations and Other | HomeBuildingMember
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	(68.3)	(61.8)	(122.5)	(110.9)
Eliminations and Other | Land [Member]
Revenues
Home sales	(297.1)	(238.7)	(561.6)	(406.2)
Cost of Goods and Services Sold	(234.1)	(204.3)	(456.7)	(352.1)
Eliminations and Other | Rental
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	0.0	(0.8)	(4.8)	(2.4)
Eliminations and Other | Financial Services [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Northwest | HomeBuildingMember
Inventories:
Total inventories	1,972.1		1,972.1		1,907.5
Revenues
Total revenues	742.1	691.1	1,320.0	1,211.5
Income (loss) before income taxes	109.3	96.4	178.8	155.0
Southwest [Member] | HomeBuildingMember
Inventories:
Total inventories	3,274.0		3,274.0		3,133.0
Revenues
Total revenues	1,283.4	920.9	2,334.8	1,723.9
Income (loss) before income taxes	171.0	81.7	305.9	165.7
South Central [Member] | HomeBuildingMember
Inventories:
Total inventories	4,085.6		4,085.6		3,810.5
Revenues
Total revenues	1,961.1	1,805.8	3,630.3	3,447.9
Income (loss) before income taxes	346.6	267.9	618.2	549.5
Southeast [Member] | HomeBuildingMember
Inventories:
Total inventories	4,378.5		4,378.5		3,958.5
Revenues
Total revenues	2,185.8	2,106.1	4,185.4	4,102.4
Income (loss) before income taxes	361.5	381.7	690.9	793.0
East [Member] | HomeBuildingMember
Inventories:
Total inventories	3,597.6		3,597.6		3,024.7
Revenues
Total revenues	1,441.8	1,206.9	2,710.1	2,350.8
Income (loss) before income taxes	254.5	183.5	459.1	373.0
North | HomeBuildingMember
Inventories:
Total inventories	2,363.5		2,363.5		2,078.0
Revenues
Total revenues	859.4	738.8	1,589.7	1,377.1
Income (loss) before income taxes	114.7	$ 56.7	200.8	$ 126.1
Corporate, Non-Segment [Member] | HomeBuildingMember
Inventories:
Total inventories	$ 258.5		$ 258.5		$ 243.6