Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2022
Assets [Abstract]
Cash and cash equivalents	$ 2,591.1		$ 2,540.5
Restricted cash	21.4		32.4
Inventories:
Construction in progress and finished homes	9,508.0		9,798.2
Residential land and lots — developed and under development	9,898.9		9,173.1
Land held for development	108.6		110.8
Land held for sale	29.5		29.4
Rental properties	2,900.3		2,544.2
Total inventories	22,445.3		21,655.7
Mortgage loans held for sale	1,782.7		2,386.0
Deferred income taxes, net	137.9		141.1
Property and equipment, net	501.7		471.6
Other assets	2,620.9		2,960.3
Goodwill	163.5		163.5
Total assets	30,264.5		30,351.1
Liabilities [Abstract]
Accounts payable	1,205.0		1,360.3
Accrued expenses and other liabilities	2,816.8		3,138.3
Notes payable	5,690.3		6,066.9
Total liabilities	9,712.1		10,565.5
Revenues
Total revenues	7,257.8	$ 7,053.4
Inventory and land option charges	27.5	4.8
Cost of sales	5,291.3	4,905.7
Selling, General and Administrative Expense	(737.0)	(665.9)
Other (income) expense	(37.7)	(15.5)
Income (loss) before income taxes	1,267.2	1,497.3
Depreciation and amortization	19.8	19.4
Cash provided by (used in) operating activities	829.1	(174.1)
HomeBuildingMember
Revenues
Home sales	6,709.2	6,656.4
Cost of Goods and Services Sold	5,057.7	4,796.2
Land [Member]
Revenues
Home sales	84.1	56.2
Cost of Goods and Services Sold	24.0	33.0
Financial Services [Member]
Revenues
Home sales	137.0	184.3
Rental
Revenues
Home sales	327.5	156.5
Cost of Goods and Services Sold	182.1	71.7
Intersegment Eliminations [Member]
Assets [Abstract]
Cash and cash equivalents	25.5		21.8
Restricted cash	0.0		0.0
Inventories:
Construction in progress and finished homes	(149.4)		(153.3)
Residential land and lots — developed and under development	(79.1)		(82.0)
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Rental properties	(25.2)		(27.9)
Total inventories	(253.7)		(263.2)
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	4.6		1.9
Property and equipment, net	117.1		97.8
Other assets	(148.6)		132.8
Goodwill	29.2		29.2
Total assets	(225.9)		20.3
Liabilities [Abstract]
Accounts payable	(422.1)		(94.8)
Accrued expenses and other liabilities	(167.3)		(214.0)
Notes payable	0.0		0.0
Total liabilities	(589.4)		(308.8)
Revenues
Total revenues	(167.4)	(374.4)
Inventory and land option charges	0.0	0.0
Cost of sales	(198.4)	(356.2)
Selling, General and Administrative Expense	(5.4)	(2.9)
Other (income) expense	9.2	5.3
Income (loss) before income taxes	16.4	(26.4)
Depreciation and amortization	2.9	3.0
Cash provided by (used in) operating activities	22.5	(56.8)
Intersegment Eliminations [Member] | HomeBuildingMember
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(49.0)	(37.7)
Intersegment Eliminations [Member] | Land [Member]
Revenues
Home sales	(167.4)	(374.4)
Cost of Goods and Services Sold	(147.7)	(317.7)
Intersegment Eliminations [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Intersegment Eliminations [Member] | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(1.7)	(0.8)
HomeBuildingMember
Assets [Abstract]
Cash and cash equivalents	2,039.4		2,040.7
Restricted cash	7.4		11.3
Inventories:
Construction in progress and finished homes	9,657.4		9,951.5
Residential land and lots — developed and under development	7,997.8		7,322.5
Land held for development	21.1		21.0
Land held for sale	29.5		29.4
Rental properties	0.0		0.0
Total inventories	17,705.8		17,324.4
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	140.4		146.3
Property and equipment, net	372.6		361.8
Other assets	2,549.4		2,266.5
Goodwill	134.3		134.3
Total assets	22,949.3		22,285.3
Liabilities [Abstract]
Accounts payable	994.3		1,149.1
Accrued expenses and other liabilities	2,494.8		2,365.7
Notes payable	2,970.0		2,942.6
Total liabilities	6,459.1		6,457.4
Revenues
Total revenues	6,744.0	6,679.4
Inventory and land option charges	24.2	3.9
Cost of sales	5,135.8	4,854.9
Selling, General and Administrative Expense	(527.1)	(497.7)
Other (income) expense	(13.3)	(6.2)
Income (loss) before income taxes	1,094.4	1,333.0
Depreciation and amortization	15.2	15.4
Cash provided by (used in) operating activities	313.9	(114.7)
HomeBuildingMember | HomeBuildingMember
Revenues
Home sales	6,709.2	6,656.4
Cost of Goods and Services Sold	5,106.7	4,833.9
HomeBuildingMember | Land [Member]
Revenues
Home sales	34.8	23.0
Cost of Goods and Services Sold	4.9	17.1
HomeBuildingMember | Financial Services [Member]
Revenues
Home sales	0.0	0.0
HomeBuildingMember | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	216.4		264.8
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	1,980.2		1,932.6
Land held for development	87.5		89.8
Land held for sale	0.0		0.0
Rental properties	0.0		0.0
Total inventories	2,067.7		2,022.4
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	5.6		5.7
Other assets	51.4		50.1
Goodwill	0.0		0.0
Total assets	2,341.1		2,343.0
Liabilities [Abstract]
Accounts payable	73.0		72.2
Accrued expenses and other liabilities	341.1		365.4
Notes payable	706.4		706.0
Total liabilities	1,120.5		1,143.6
Revenues
Total revenues	216.7	407.6
Inventory and land option charges	2.4	0.6
Cost of sales	169.2	334.2
Selling, General and Administrative Expense	(22.9)	(21.5)
Other (income) expense	(3.3)	(1.6)
Income (loss) before income taxes	27.9	53.5
Depreciation and amortization	0.7	0.5
Cash provided by (used in) operating activities	(49.8)	5.8
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	216.7	407.6
Cost of Goods and Services Sold	166.8	333.6
Forestar Group [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Forestar Group [Member] | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	198.4		103.3
Restricted cash	12.2		19.7
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	1,782.7		2,386.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	4.2		4.3
Other assets	143.1		492.5
Goodwill	0.0		0.0
Total assets	2,140.6		3,005.8
Liabilities [Abstract]
Accounts payable	0.0		0.2
Accrued expenses and other liabilities	123.7		596.2
Notes payable	1,213.9		1,618.3
Total liabilities	1,337.6		2,214.7
Revenues
Total revenues	137.0	184.3
Inventory and land option charges	0.0	0.0
Cost of sales	0.0	0.0
Selling, General and Administrative Expense	(134.1)	(125.3)
Other (income) expense	(15.3)	(8.1)
Income (loss) before income taxes	18.2	67.1
Depreciation and amortization	0.5	0.4
Cash provided by (used in) operating activities	493.1	247.5
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] | Financial Services [Member]
Revenues
Home sales	137.0	184.3
Financial Services [Member] | Rental
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Rental
Assets [Abstract]
Cash and cash equivalents	111.4		109.9
Restricted cash	1.8		1.4
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	2,925.5		2,572.1
Total inventories	2,925.5		2,572.1
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	(7.1)		(7.1)
Property and equipment, net	2.2		2.0
Other assets	25.6		18.4
Goodwill	0.0		0.0
Total assets	3,059.4		2,696.7
Liabilities [Abstract]
Accounts payable	559.8		233.6
Accrued expenses and other liabilities	24.5		25.0
Notes payable	800.0		800.0
Total liabilities	1,384.3		1,058.6
Revenues
Total revenues	327.5	156.5
Inventory and land option charges	0.9	0.3
Cost of sales	184.7	72.8
Selling, General and Administrative Expense	(47.5)	(18.5)
Other (income) expense	(15.0)	(4.9)
Income (loss) before income taxes	110.3	70.1
Depreciation and amortization	0.5	0.1
Cash provided by (used in) operating activities	49.4	(255.9)
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 | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Rental | Rental
Revenues
Home sales	327.5	156.5
Cost of Goods and Services Sold	183.8	72.5
Northwest | HomeBuildingMember
Inventories:
Total inventories	1,873.2		1,802.2
Revenues
Total revenues	520.4	569.0
Income (loss) before income taxes	58.7	111.8
Southwest [Member] | HomeBuildingMember
Inventories:
Total inventories	2,941.9		2,801.7
Revenues
Total revenues	803.0	911.6
Income (loss) before income taxes	84.0	159.3
South Central [Member] | HomeBuildingMember
Inventories:
Total inventories	3,921.2		3,931.7
Revenues
Total revenues	1,642.1	1,694.3
Income (loss) before income taxes	281.6	354.3
Southeast [Member] | HomeBuildingMember
Inventories:
Total inventories	4,056.6		4,091.1
Revenues
Total revenues	1,996.3	1,810.9
Income (loss) before income taxes	411.3	415.4
East [Member] | HomeBuildingMember
Inventories:
Total inventories	2,632.0		2,542.7
Revenues
Total revenues	1,143.9	1,074.9
Income (loss) before income taxes	189.4	202.3
North | HomeBuildingMember
Inventories:
Total inventories	2,050.3		1,935.7
Revenues
Total revenues	638.3	618.7
Income (loss) before income taxes	69.4	$ 89.9
Corporate, Non-Segment [Member] | HomeBuildingMember
Inventories:
Total inventories	$ 230.6		$ 219.3