Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Assets [Abstract]
Cash and cash equivalents	$ 3,323.4		$ 3,873.6
Restricted cash	21.2		26.5
Inventories:
Construction in progress and finished homes	9,467.7		9,001.4
Residential land and lots — developed and under development	11,497.1		10,621.9
Land held for development	134.5		50.0
Land held for sale	3.5		8.7
Rental properties	2,947.5		2,691.3
Total inventories	24,050.3		22,373.3
Mortgage loans held for sale	2,044.1		2,519.9
Deferred income taxes, net	175.9		187.2
Property and equipment, net	475.5		445.4
Other assets	3,127.7		2,993.0
Goodwill	163.5		163.5
Total assets	33,381.6		32,582.4
Liabilities [Abstract]
Accounts payable	1,260.3		1,246.2
Accrued expenses and other liabilities	3,227.4		3,103.8
Notes payable	5,290.4		5,094.5
Total liabilities	9,778.1		9,444.5
Revenues
Total revenues	7,726.0	$ 7,257.8
Inventory and land option charges	6.1	27.5
Cost of sales	5,719.8	5,291.3
Selling, General and Administrative Expense	(835.0)	(737.0)
Other (income) expense	(76.3)	(37.7)
Income (loss) before income taxes	1,247.5	1,267.2
Depreciation and amortization	20.0	19.8
Net cash (used in) provided by operating activities	(153.4)	829.1
HomeBuildingMember
Revenues
Home sales	7,276.4	6,709.2
Cost of Goods and Services Sold	5,553.8	5,057.7
Land [Member]
Revenues
Home sales	61.7	84.1
Cost of Goods and Services Sold	23.4	24.0
Rental
Revenues
Home sales	195.3	327.5
Cost of Goods and Services Sold	136.5	182.1
Financial Services [Member]
Revenues
Home sales	192.6	137.0
HomeBuildingMember
Assets [Abstract]
Cash and cash equivalents	2,504.1		2,920.2
Restricted cash	6.0		6.5
Inventories:
Construction in progress and finished homes	9,602.5		9,134.3
Residential land and lots — developed and under development	9,740.5		8,992.3
Land held for development	20.4		20.5
Land held for sale	3.5		8.7
Rental properties	0.0		0.0
Total inventories	19,366.9		18,155.8
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	218.0		229.8
Property and equipment, net	444.8		415.0
Other assets	2,670.6		2,838.5
Goodwill	134.3		134.3
Total assets	25,344.7		24,700.1
Liabilities [Abstract]
Accounts payable	1,017.0		1,033.7
Accrued expenses and other liabilities	2,761.4		2,585.5
Notes payable	2,355.4		2,329.9
Total liabilities	6,133.8		5,949.1
Revenues
Total revenues	7,296.7	6,744.0
Inventory and land option charges	5.5	24.2
Cost of sales	5,626.7	5,135.8
Selling, General and Administrative Expense	(603.4)	(527.1)
Other (income) expense	(29.5)	(13.3)
Income (loss) before income taxes	1,096.1	1,094.4
Depreciation and amortization	18.0	15.2
Net cash (used in) provided by operating activities	31.2	313.9
HomeBuildingMember | HomeBuildingMember
Revenues
Home sales	7,276.4	6,709.2
Cost of Goods and Services Sold	5,608.0	5,106.7
HomeBuildingMember | Land [Member]
Revenues
Home sales	20.3	34.8
Cost of Goods and Services Sold	13.2	4.9
HomeBuildingMember | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
HomeBuildingMember | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Rental
Assets [Abstract]
Cash and cash equivalents	117.7		136.1
Restricted cash	2.2		3.3
Inventories:
Construction in progress and finished homes	0.0		0.0
Residential land and lots — developed and under development	0.0		0.0
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Rental properties	2,962.7		2,708.4
Total inventories	2,962.7		2,708.4
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	(19.9)		(19.9)
Property and equipment, net	2.4		2.4
Other assets	35.6		29.8
Goodwill	0.0		0.0
Total assets	3,100.7		2,860.1
Liabilities [Abstract]
Accounts payable	424.0		698.6
Accrued expenses and other liabilities	34.6		43.2
Notes payable	950.0		400.0
Total liabilities	1,408.6		1,141.8
Revenues
Total revenues	195.3	327.5
Inventory and land option charges	0.4	0.9
Cost of sales	141.7	184.7
Selling, General and Administrative Expense	(47.4)	(47.5)
Other (income) expense	(25.1)	(15.0)
Income (loss) before income taxes	31.3	110.3
Depreciation and amortization	0.6	0.5
Net cash (used in) provided by operating activities	(516.2)	49.4
Rental | HomeBuildingMember
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Rental | Land [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Rental | Rental
Revenues
Home sales	195.3	327.5
Cost of Goods and Services Sold	141.3	183.8
Rental | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	458.9		616.0
Restricted cash	0.0		0.0
Inventories:
Construction in progress and finished homes	0.0		0.0
Residential land and lots — developed and under development	1,895.7		1,760.8
Land held for development	114.1		29.5
Land held for sale	0.0		0.0
Rental properties	0.0		0.0
Total inventories	2,009.8		1,790.3
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	5.8		5.9
Other assets	59.3		58.5
Goodwill	0.0		0.0
Total assets	2,533.8		2,470.7
Liabilities [Abstract]
Accounts payable	65.3		68.4
Accrued expenses and other liabilities	354.4		337.4
Notes payable	705.3		695.0
Total liabilities	1,125.0		1,100.8
Revenues
Total revenues	305.9	216.7
Inventory and land option charges	0.2	2.4
Cost of sales	233.0	169.2
Selling, General and Administrative Expense	(28.0)	(22.9)
Other (income) expense	(6.3)	(3.3)
Income (loss) before income taxes	51.2	27.9
Depreciation and amortization	0.8	0.7
Net cash (used in) provided by operating activities	(156.7)	(49.8)
Forestar Group [Member] | HomeBuildingMember
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Forestar Group [Member] | Land [Member]
Revenues
Home sales	305.9	216.7
Cost of Goods and Services Sold	232.8	166.8
Forestar Group [Member] | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Forestar Group [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	226.9		189.1
Restricted cash	13.0		16.7
Inventories:
Construction in progress and finished homes	0.0		0.0
Residential land and lots — developed and under development	0.0		0.0
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Rental properties	0.0		0.0
Total inventories	0.0		0.0
Mortgage loans held for sale	2,044.1		2,519.9
Deferred income taxes, net	0.0		0.0
Property and equipment, net	4.3		4.1
Other assets	386.9		250.3
Goodwill	0.0		0.0
Total assets	2,675.2		2,980.1
Liabilities [Abstract]
Accounts payable	0.0		0.1
Accrued expenses and other liabilities	354.4		280.4
Notes payable	1,279.7		1,669.6
Total liabilities	1,634.1		1,950.1
Revenues
Total revenues	192.6	137.0
Inventory and land option charges	0.0	0.0
Cost of sales	0.0	0.0
Selling, General and Administrative Expense	(151.5)	(134.1)
Other (income) expense	(24.9)	(15.3)
Income (loss) before income taxes	66.0	18.2
Depreciation and amortization	0.5	0.5
Net cash (used in) provided by operating activities	464.7	493.1
Financial Services [Member] | HomeBuildingMember
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Financial Services [Member] | Land [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Financial Services [Member] | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Financial Services [Member] | Financial Services [Member]
Revenues
Home sales	192.6	137.0
Eliminations and Other
Assets [Abstract]
Cash and cash equivalents	15.8		12.2
Restricted cash	0.0		0.0
Inventories:
Construction in progress and finished homes	(134.8)		(132.9)
Residential land and lots — developed and under development	(139.1)		(131.2)
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Rental properties	(15.2)		(17.1)
Total inventories	(289.1)		(281.2)
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	(22.2)		(22.7)
Property and equipment, net	18.2		18.0
Other assets	(24.7)		(184.1)
Goodwill	29.2		29.2
Total assets	(272.8)		(428.6)
Liabilities [Abstract]
Accounts payable	(246.0)		(554.6)
Accrued expenses and other liabilities	(277.4)		(142.7)
Notes payable	0.0		0.0
Total liabilities	(523.4)		(697.3)
Revenues
Total revenues	(264.5)	(167.4)
Inventory and land option charges	0.0	0.0
Cost of sales	(281.6)	(198.4)
Selling, General and Administrative Expense	(4.7)	(5.4)
Other (income) expense	9.5	9.2
Income (loss) before income taxes	2.9	16.4
Depreciation and amortization	0.1	2.9
Net cash (used in) provided by operating activities	23.6	22.5
Eliminations and Other | HomeBuildingMember
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(54.2)	(49.0)
Eliminations and Other | Land [Member]
Revenues
Home sales	(264.5)	(167.4)
Cost of Goods and Services Sold	(222.6)	(147.7)
Eliminations and Other | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(4.8)	(1.7)
Eliminations and Other | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Northwest | HomeBuildingMember
Inventories:
Total inventories	1,990.0		1,907.5
Revenues
Total revenues	577.9	520.4
Income (loss) before income taxes	69.5	58.7
Southwest [Member] | HomeBuildingMember
Inventories:
Total inventories	3,311.4		3,133.0
Revenues
Total revenues	1,051.4	803.0
Income (loss) before income taxes	134.9	84.0
South Central [Member] | HomeBuildingMember
Inventories:
Total inventories	3,950.6		3,810.5
Revenues
Total revenues	1,669.2	1,642.1
Income (loss) before income taxes	271.6	281.6
Southeast [Member] | HomeBuildingMember
Inventories:
Total inventories	4,186.5		3,958.5
Revenues
Total revenues	1,999.6	1,996.3
Income (loss) before income taxes	329.4	411.3
East [Member] | HomeBuildingMember
Inventories:
Total inventories	3,367.4		3,024.7
Revenues
Total revenues	1,268.3	1,143.9
Income (loss) before income taxes	204.6	189.4
North | HomeBuildingMember
Inventories:
Total inventories	2,302.3		2,078.0
Revenues
Total revenues	730.3	638.3
Income (loss) before income taxes	86.1	$ 69.4
Corporate, Non-Segment [Member] | HomeBuildingMember
Inventories:
Total inventories	$ 258.7		$ 243.6