Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Assets [Abstract]
Cash and cash equivalents	$ 2,442.1		$ 3,210.4
Restricted cash	22.5		26.8
Inventories:
Construction in progress and finished homes	8,742.9		7,739.2
Residential land and lots — developed and under development	8,162.4		7,781.8
Land held for development	131.4		110.9
Land held for sale	24.8		25.4
Rental properties	1,141.0		821.8
Total inventories	18,202.5		16,479.1
Mortgage loans held for sale	1,833.3		2,027.3
Deferred income taxes, net	137.6		155.3
Property and equipment, net	410.4		392.9
Other assets	1,787.5		1,560.6
Goodwill	163.5		163.5
Total assets	24,999.4		24,015.9
Liabilities [Abstract]
Accounts payable	1,150.7		1,177.0
Accrued expenses and other liabilities	2,580.0		2,210.3
Notes payable	5,255.3		5,412.4
Total liabilities	8,986.0		8,799.7
Revenues
Total revenues	7,053.4	$ 5,933.4
Inventory and land option charges	4.8	8.3
Cost of sales	4,905.7	4,332.5
Selling, General and Administrative Expense	665.9	585.9
Gain on sale of assets	0.0	(14.0)
Other (income) expense	(15.5)	(5.3)
Income (loss) before income taxes	1,497.3	1,034.3
Depreciation and amortization	19.4	22.9
Cash provided by (used in) operating activities	(174.1)	(252.1)
HomeBuildingMember
Revenues
Home sales	6,656.4	5,698.7
Cost of Goods and Services Sold	4,796.2	4,297.4
Land [Member]
Revenues
Home sales	56.2	47.5
Cost of Goods and Services Sold	33.0	26.8
Financial Services [Member]
Revenues
Home sales	184.3	187.2
Rental
Revenues
Home sales	156.5	0.0
Cost of Goods and Services Sold	71.7	0.0
Intersegment Eliminations [Member]
Assets [Abstract]
Cash and cash equivalents	13.8		10.9
Restricted cash	0.0		0.0
Inventories:
Construction in progress and finished homes	(136.6)		(108.8)
Residential land and lots — developed and under development	(58.9)		(46.9)
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Rental properties	(20.1)		(19.1)
Total inventories	(215.6)		(174.8)
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	(4.0)		(3.9)
Property and equipment, net	86.7		82.6
Other assets	(39.8)		(62.0)
Goodwill	29.2		29.2
Total assets	(129.7)		(118.0)
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	(166.7)		(168.5)
Notes payable	(0.6)		(0.7)
Total liabilities	(167.3)		(169.2)
Revenues
Total revenues	(374.4)	(309.3)
Inventory and land option charges	0.0	0.0
Cost of sales	(356.2)	(294.9)
Selling, General and Administrative Expense	2.9	2.2
Gain on sale of assets		(14.0)
Other (income) expense	5.3	7.6
Income (loss) before income taxes	(26.4)	(10.2)
Depreciation and amortization	3.0	2.7
Cash provided by (used in) operating activities	(56.8)	62.0
Intersegment Eliminations [Member] | HomeBuildingMember
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(37.7)	(27.7)
Intersegment Eliminations [Member] | Land [Member]
Revenues
Home sales	(374.4)	(277.5)
Cost of Goods and Services Sold	(317.7)	(249.4)
Intersegment Eliminations [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Intersegment Eliminations [Member] | Rental
Revenues
Home sales	0.0	(31.8)
Cost of Goods and Services Sold	(0.8)	(17.8)
HomeBuildingMember
Assets [Abstract]
Cash and cash equivalents	2,133.4		2,950.1
Restricted cash	12.4		8.4
Inventories:
Construction in progress and finished homes	8,879.5		7,848.0
Residential land and lots — developed and under development	6,366.7		6,004.0
Land held for development	25.9		30.4
Land held for sale	24.8		25.4
Rental properties	0.0		0.0
Total inventories	15,296.9		13,907.8
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	141.6		159.2
Property and equipment, net	315.5		303.3
Other assets	1,689.3		1,468.7
Goodwill	134.3		134.3
Total assets	19,723.4		18,931.8
Liabilities [Abstract]
Accounts payable	1,021.5		1,073.7
Accrued expenses and other liabilities	2,316.4		1,941.3
Notes payable	3,291.0		3,214.0
Total liabilities	6,628.9		6,229.0
Revenues
Total revenues	6,679.4	5,716.6
Inventory and land option charges	3.9	7.9
Cost of sales	4,854.9	4,346.7
Selling, General and Administrative Expense	497.7	449.4
Gain on sale of assets		0.0
Other (income) expense	(6.2)	(2.1)
Income (loss) before income taxes	1,333.0	922.6
Depreciation and amortization	15.4	16.4
Cash provided by (used in) operating activities	(114.7)	(269.6)
HomeBuildingMember | HomeBuildingMember
Revenues
Home sales	6,656.4	5,698.7
Cost of Goods and Services Sold	4,833.9	4,325.1
HomeBuildingMember | Land [Member]
Revenues
Home sales	23.0	17.9
Cost of Goods and Services Sold	17.1	13.7
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	162.5		153.6
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	1,854.6		1,824.7
Land held for development	105.5		80.5
Land held for sale	0.0		0.0
Rental properties	0.0		0.0
Total inventories	1,960.1		1,905.2
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	4.2		2.9
Other assets	35.7		40.0
Goodwill	0.0		0.0
Total assets	2,162.5		2,101.7
Liabilities [Abstract]
Accounts payable	53.5		47.4
Accrued expenses and other liabilities	345.7		333.9
Notes payable	704.9		704.5
Total liabilities	1,104.1		1,085.8
Revenues
Total revenues	407.6	307.1
Inventory and land option charges	0.6	0.4
Cost of sales	334.2	262.9
Selling, General and Administrative Expense	21.5	15.5
Gain on sale of assets		0.0
Other (income) expense	(1.6)	(0.5)
Income (loss) before income taxes	53.5	29.2
Depreciation and amortization	0.5	0.7
Cash provided by (used in) operating activities	5.8	(158.7)
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	407.6	307.1
Cost of Goods and Services Sold	333.6	262.5
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	88.2		79.0
Restricted cash	9.5		18.0
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	1,833.3		2,027.3
Deferred income taxes, net	0.0		0.0
Property and equipment, net	3.3		3.5
Other assets	93.3		107.6
Goodwill	0.0		0.0
Total assets	2,027.6		2,235.4
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	75.7		88.6
Notes payable	1,260.0		1,494.6
Total liabilities	1,335.7		1,583.2
Revenues
Total revenues	184.3	187.2
Inventory and land option charges	0.0	0.0
Cost of sales	0.0	0.0
Selling, General and Administrative Expense	125.3	109.5
Gain on sale of assets		0.0
Other (income) expense	(8.1)	(6.4)
Income (loss) before income taxes	67.1	84.1
Depreciation and amortization	0.4	0.4
Cash provided by (used in) operating activities	247.5	173.1
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] | Financial Services [Member]
Revenues
Home sales	184.3	187.2
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	44.2		16.8
Restricted cash	0.6		0.4
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	1,161.1		840.9
Total inventories	1,161.1		840.9
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	0.7		0.6
Other assets	9.0		6.3
Goodwill	0.0		0.0
Total assets	1,215.6		865.0
Liabilities [Abstract]
Accounts payable	75.7		55.9
Accrued expenses and other liabilities	8.9		15.0
Notes payable	0.0		0.0
Total liabilities	84.6		70.9
Revenues
Total revenues	156.5	31.8
Inventory and land option charges	0.3	0.0
Cost of sales	72.8	17.8
Selling, General and Administrative Expense	18.5	9.3
Gain on sale of assets		0.0
Other (income) expense	(4.9)	(3.9)
Income (loss) before income taxes	70.1	8.6
Depreciation and amortization	0.1	2.7
Cash provided by (used in) operating activities	(255.9)	(58.9)
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	156.5	31.8
Cost of Goods and Services Sold	72.5	17.8
Northwest | HomeBuildingMember
Inventories:
Total inventories	1,450.8		1,307.5
Revenues
Total revenues	569.0	548.0
Income (loss) before income taxes	111.8	86.8
Southwest [Member] | HomeBuildingMember
Inventories:
Total inventories	2,743.3		2,445.6
Revenues
Total revenues	911.6	838.6
Income (loss) before income taxes	159.3	115.2
South Central [Member] | HomeBuildingMember
Inventories:
Total inventories	3,772.2		3,479.3
Revenues
Total revenues	1,694.3	1,363.5
Income (loss) before income taxes	354.3	240.0
Southeast [Member] | HomeBuildingMember
Inventories:
Total inventories	3,444.9		3,178.6
Revenues
Total revenues	1,810.9	1,465.0
Income (loss) before income taxes	415.4	247.2
East [Member] | HomeBuildingMember
Inventories:
Total inventories	2,118.3		1,919.6
Revenues
Total revenues	1,074.9	1,007.7
Income (loss) before income taxes	202.3	170.7
North | HomeBuildingMember
Inventories:
Total inventories	1,558.4		1,368.9
Revenues
Total revenues	618.7	493.8
Income (loss) before income taxes	89.9	$ 62.7
Corporate, Non-Segment [Member] | HomeBuildingMember
Inventories:
Total inventories	$ 209.0		$ 208.3