Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Assets [Abstract]
Cash and cash equivalents	$ 2,205.2		$ 2,205.2		$ 3,018.5
Restricted cash	26.7		26.7		21.6
Inventories:
Construction in progress and finished homes	7,300.1		7,300.1		5,984.1
Residential land and lots — developed and under development	7,066.1		7,066.1		6,171.8
Land held for development	86.0		86.0		53.2
Land held for sale	24.1		24.1		28.3
Total inventories	14,476.3		14,476.3		12,237.4
Mortgage loans held for sale	1,755.8		1,755.8		1,529.0
Deferred income taxes, net	142.8		142.8		144.9
Property and equipment, net	895.0		895.0		683.7
Other assets	1,418.5		1,418.5		1,113.7
Goodwill	163.5		163.5		163.5
Total assets	21,083.8		21,083.8		18,912.3
Liabilities [Abstract]
Accounts payable	1,137.4		1,137.4		900.5
Accrued expenses and other liabilities	2,198.1		2,198.1		1,607.0
Notes payable	4,472.3		4,472.3		4,283.3
Total liabilities	7,807.8		7,807.8		6,790.8
Revenues
Total revenues	6,446.9	$ 4,500.0	12,380.3	$ 8,520.7
Inventory and land option charges	3.8	8.9	12.1	12.8
Cost of sales	4,650.9	3,450.8	8,983.5	6,535.0
Selling, General and Administrative Expense	621.5	466.8	1,207.4	922.6
Gain on sale of assets	0.0	(28.5)	(14.0)	(59.5)
Other (income) expense	(5.4)	(10.4)	(10.8)	(21.9)
Income (loss) before income taxes	1,179.9	621.3	2,214.2	1,144.5
Depreciation and amortization			39.3	38.8
Net cash used in operating activities			(154.9)	(395.1)
Intersegment Eliminations [Member]
Assets [Abstract]
Cash and cash equivalents	0.0		0.0		0.0
Restricted cash	0.0		0.0		0.0
Inventories:
Construction in progress and finished homes	(77.3)		(77.3)		(53.4)
Residential land and lots — developed and under development	(41.4)		(41.4)		(33.9)
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Total inventories	(118.7)		(118.7)		(87.3)
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	(0.7)		(0.7)		2.6
Property and equipment, net	(6.1)		(6.1)		(3.0)
Other assets	(105.5)		(105.5)		(96.1)
Goodwill	29.2		29.2		29.2
Total assets	(201.8)		(201.8)		(154.6)
Liabilities [Abstract]
Accounts payable	0.0		0.0		0.0
Accrued expenses and other liabilities	(151.7)		(151.7)		(128.1)
Notes payable	(2.7)		(2.7)		(4.8)
Total liabilities	(154.4)		(154.4)		(132.9)
Revenues
Total revenues	(268.3)	(151.9)	(562.5)	(373.2)
Inventory and land option charges	0.0	0.0	0.0	0.0
Cost of sales	(250.8)	(141.4)	(528.0)	(342.3)
Selling, General and Administrative Expense	0.1	0.1	0.2	0.2
Gain on sale of assets		0.0	0.0	0.0
Other (income) expense	0.0	0.0	(0.7)	0.0
Income (loss) before income taxes	(17.6)	(10.6)	(34.0)	(31.1)
Depreciation and amortization			0.3	0.3
Net cash used in operating activities			(15.3)	(16.8)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	94.7		94.7		55.6
Restricted cash	12.4		12.4		11.9
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
Total inventories	0.0		0.0		0.0
Mortgage loans held for sale	1,755.8		1,755.8		1,529.0
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	3.5		3.5		3.9
Other assets	188.9		188.9		125.8
Goodwill	0.0		0.0		0.0
Total assets	2,055.3		2,055.3		1,726.2
Liabilities [Abstract]
Accounts payable	0.0		0.0		0.0
Accrued expenses and other liabilities	195.4		195.4		86.8
Notes payable	1,203.5		1,203.5		1,132.6
Total liabilities	1,398.9		1,398.9		1,219.4
Revenues
Total revenues	225.1	104.5	412.3	207.4
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	123.7	85.9	233.3	163.8
Gain on sale of assets		0.0	0.0	0.0
Other (income) expense	(6.3)	(6.1)	(12.8)	(11.6)
Income (loss) before income taxes	107.7	24.7	191.8	55.2
Depreciation and amortization			0.8	0.8
Net cash used in operating activities			(32.3)	(312.1)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	167.2		167.2		394.3
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	1,643.1		1,643.1		1,304.3
Land held for development	55.5		55.5		5.4
Land held for sale	0.0		0.0		0.0
Total inventories	1,698.6		1,698.6		1,309.7
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	2.0		2.0		1.1
Other assets	31.1		31.1		34.8
Goodwill	0.0		0.0		0.0
Total assets	1,898.9		1,898.9		1,739.9
Liabilities [Abstract]
Accounts payable	39.8		39.8		29.2
Accrued expenses and other liabilities	260.0		260.0		197.8
Notes payable	654.6		654.6		641.1
Total liabilities	954.4		954.4		868.1
Revenues
Total revenues	287.1	159.1	594.2	406.4
Inventory and land option charges	0.6	0.1	0.9	0.4
Cost of sales	233.8	136.6	496.7	353.2
Selling, General and Administrative Expense	16.3	11.2	31.8	21.7
Gain on sale of assets		(0.3)	0.0	(0.1)
Other (income) expense	(0.6)	(2.1)	(1.1)	(4.2)
Income (loss) before income taxes	37.6	13.7	66.8	35.8
Depreciation and amortization			0.2	0.1
Net cash used in operating activities			(249.5)	(123.8)
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	56.1		56.1		17.5
Restricted cash	0.3		0.3		0.2
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
Total inventories	0.0		0.0		0.0
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	432.4		432.4		308.9
Other assets	58.7		58.7		52.8
Goodwill	0.0		0.0		0.0
Total assets	547.5		547.5		379.4
Liabilities [Abstract]
Accounts payable	19.6		19.6		12.0
Accrued expenses and other liabilities	13.2		13.2		12.2
Notes payable	0.0		0.0		0.0
Total liabilities	32.8		32.8		24.2
Revenues
Total revenues	15.3	9.5	30.0	18.3
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	11.9	7.8	21.5	16.7
Gain on sale of assets		(28.2)	(0.9)	(59.4)
Other (income) expense	3.3	1.9	7.1	3.5
Income (loss) before income taxes	0.1	28.0	2.3	57.5
Depreciation and amortization			8.0	3.9
Net cash used in operating activities			4.2	5.5
HomeBuildingMember
Assets [Abstract]
Cash and cash equivalents	1,887.2		1,887.2		2,551.1
Restricted cash	14.0		14.0		9.5
Inventories:
Construction in progress and finished homes	7,377.4		7,377.4		6,037.5
Residential land and lots — developed and under development	5,464.4		5,464.4		4,901.4
Land held for development	30.5		30.5		47.8
Land held for sale	24.1		24.1		28.3
Total inventories	12,896.4		12,896.4		11,015.0
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	143.5		143.5		142.3
Property and equipment, net	463.2		463.2		372.8
Other assets	1,245.3		1,245.3		996.4
Goodwill	134.3		134.3		134.3
Total assets	16,783.9		16,783.9		15,221.4
Liabilities [Abstract]
Accounts payable	1,078.0		1,078.0		859.3
Accrued expenses and other liabilities	1,881.2		1,881.2		1,438.3
Notes payable	2,616.9		2,616.9		2,514.4
Total liabilities	5,576.1		5,576.1		4,812.0
Revenues
Total revenues	6,187.7	4,378.8	11,906.3	8,261.8
Inventory and land option charges	3.2	8.8	11.2	12.4
Cost of sales	4,667.9	3,455.6	9,014.8	6,524.1
Selling, General and Administrative Expense	469.5	361.8	920.6	720.2
Gain on sale of assets		0.0	(13.1)	0.0
Other (income) expense	(1.8)	(4.1)	(3.3)	(9.6)
Income (loss) before income taxes	1,052.1	565.5	1,987.3	1,027.1
Depreciation and amortization			30.0	33.7
Net cash used in operating activities			138.0	52.1
East [Member] | HomeBuildingMember
Inventories:
Total inventories	1,518.8		1,518.8		1,328.3
Revenues
Total revenues	861.5	579.7	1,670.7	1,100.2
Inventory and land option charges	0.7	(0.5)	1.3	(0.3)
Income (loss) before income taxes	146.0	73.3	278.5	133.2
Midwest [Member] | HomeBuildingMember
Inventories:
Total inventories	1,113.5		1,113.5		958.5
Revenues
Total revenues	468.4	308.7	881.5	591.3
Inventory and land option charges	0.2	1.4	0.3	1.5
Income (loss) before income taxes	60.9	23.6	110.1	42.3
Southeast [Member] | HomeBuildingMember
Inventories:
Total inventories	3,305.9		3,305.9		2,919.9
Revenues
Total revenues	2,054.9	1,316.7	3,833.3	2,467.3
Inventory and land option charges	1.1	2.8	7.8	4.2
Income (loss) before income taxes	376.7	187.8	682.5	333.9
South Central [Member] | HomeBuildingMember
Inventories:
Total inventories	3,477.8		3,477.8		2,879.9
Revenues
Total revenues	1,504.1	1,066.4	2,979.0	2,025.1
Inventory and land option charges	0.4	2.9	0.6	4.4
Income (loss) before income taxes	266.8	156.2	531.5	288.9
Southwest [Member] | HomeBuildingMember
Inventories:
Total inventories	888.1		888.1		695.8
Revenues
Total revenues	243.1	199.7	475.7	410.7
Inventory and land option charges	0.3	0.0	0.4	0.1
Income (loss) before income taxes	39.5	30.8	78.1	65.3
West [Member] | HomeBuildingMember
Inventories:
Total inventories	2,376.9		2,376.9		2,009.1
Revenues
Total revenues	1,055.7	907.6	2,066.1	1,667.2
Inventory and land option charges	0.5	2.2	0.8	2.5
Income (loss) before income taxes	162.2	93.8	306.6	163.5
Corporate, Non-Segment [Member] | HomeBuildingMember
Inventories:
Total inventories	215.4		215.4		223.5
Multifamily [Member] | Other Segments [Member]
Inventories:
Total assets	394.4		394.4		$ 246.2
Land [Member]
Revenues
Home sales	51.4	32.2	98.9	86.7
Cost of Goods and Services Sold	30.7	17.4	57.6	52.9
Land [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	(268.3)	(151.9)	(562.5)	(373.2)
Cost of Goods and Services Sold	(215.2)	(130.4)	(464.7)	(324.6)
Land [Member] | Financial Services [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
Land [Member] | Forestar Group [Member]
Revenues
Home sales	287.1	159.1	594.2	406.4
Cost of Goods and Services Sold	233.2	136.5	495.8	352.8
Land [Member] | Other Segments [Member]
Revenues
Home sales	15.3	9.5	30.0	18.3
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Land [Member] | HomeBuildingMember
Revenues
Home sales	17.3	15.5	37.2	35.2
Cost of Goods and Services Sold	12.7	11.3	26.5	24.7
HomeBuildingMember
Revenues
Home sales	6,170.4	4,363.3	11,869.1	8,226.6
Cost of Goods and Services Sold	4,616.4	3,424.5	8,913.8	6,469.3
HomeBuildingMember | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	(35.6)	(11.0)	(63.3)	(17.7)
HomeBuildingMember | Financial Services [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
HomeBuildingMember | Forestar Group [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
HomeBuildingMember | Other Segments [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
HomeBuildingMember | HomeBuildingMember
Revenues
Home sales	6,170.4	4,363.3	11,869.1	8,226.6
Cost of Goods and Services Sold	4,652.0	3,435.5	8,977.1	6,487.0
Financial Services [Member]
Revenues
Home sales	225.1	104.5	412.3	207.4
Financial Services [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Services [Member] | Financial Services [Member]
Revenues
Home sales	225.1	104.5	412.3	207.4
Financial Services [Member] | Forestar Group [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Services [Member] | Other Segments [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Services [Member] | HomeBuildingMember
Revenues
Home sales	$ 0.0	$ 0.0	$ 0.0	$ 0.0