Segment Information (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020
Assets [Abstract]
Cash and cash equivalents	$ 1,942.7			$ 1,942.7		$ 3,018.5
Restricted cash	26.4			26.4		21.6
Inventories:
Construction in progress and finished homes	7,744.1			7,744.1		5,984.1
Residential land and lots — developed and under development	7,407.9			7,407.9		6,171.8
Land held for development	113.9			113.9		53.2
Land held for sale	23.5			23.5		28.3
Rental properties	722.3			722.3		0.0
Total inventories	16,011.7			16,011.7		12,237.4
Mortgage loans held for sale	1,644.1			1,644.1		1,529.0
Deferred income taxes, net	147.1			147.1		144.9
Property and equipment, net	367.0			367.0		683.7
Other assets	1,491.0			1,491.0		1,113.7
Goodwill	163.5			163.5		163.5
Total assets	21,793.5			21,793.5		18,912.3
Liabilities [Abstract]
Accounts payable	1,250.9			1,250.9		900.5
Accrued expenses and other liabilities	2,001.3			2,001.3		1,607.0
Notes payable	4,416.3			4,416.3		4,283.3
Total liabilities	7,668.5			7,668.5		6,790.8
Revenues
Total revenues	7,284.6		$ 5,390.0	19,664.9	$ 13,910.8
Inventory and land option charges	5.6		5.0	17.6	17.8
Cost of sales	5,212.6		4,084.7	14,196.0	10,619.7
Selling, General and Administrative Expense	655.7		527.5	1,863.2	1,450.1
Gain on sale of assets	0.0		0.0	(14.0)	(59.5)
Loss on extinguishment of debt	18.1		0.0	18.1	0.0
Other (income) expense	(17.4)		(4.6)	(28.2)	(26.4)
Income (loss) before income taxes	1,415.6		782.4	3,629.8	1,926.9
Depreciation and amortization				56.7	57.4
Cash provided by (used in) operating activities				(34.5)	588.9
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	(93.7)			(93.7)		(53.4)
Residential land and lots — developed and under development	(45.4)			(45.4)		(33.9)
Land held for development	0.0			0.0		0.0
Land held for sale	0.0			0.0		0.0
Rental properties	(17.1)			(17.1)
Total inventories	(156.2)			(156.2)		(87.3)
Mortgage loans held for sale	0.0			0.0		0.0
Deferred income taxes, net	(5.4)			(5.4)		2.6
Property and equipment, net	0.0			0.0		(3.0)
Other assets	(111.2)			(111.2)		(96.1)
Goodwill	29.2			29.2		29.2
Total assets	(243.6)			(243.6)		(154.6)
Liabilities [Abstract]
Accounts payable	0.0			0.0		0.0
Accrued expenses and other liabilities	(172.0)			(172.0)		(128.1)
Notes payable	(1.4)			(1.4)		(4.8)
Total liabilities	(173.4)			(173.4)		(132.9)
Revenues
Total revenues	(303.2)		(175.5)	(865.8)	(548.6)
Inventory and land option charges	0.0		0.0	0.0	0.0
Cost of sales	(283.0)		(169.8)	(811.0)	(512.0)
Selling, General and Administrative Expense	0.2		0.2	0.5	0.4
Gain on sale of assets				0.0	0.0
Loss on extinguishment of debt	0.0			0.0
Other (income) expense	0.0		0.0	(0.8)	0.0
Income (loss) before income taxes	(20.4)		(5.9)	(54.5)	(37.0)
Depreciation and amortization				0.4	0.4
Cash provided by (used in) operating activities				(8.0)	(16.8)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	101.1			101.1		55.6
Restricted cash	14.6			14.6		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
Rental properties	0.0			0.0
Total inventories	0.0			0.0		0.0
Mortgage loans held for sale	1,644.1			1,644.1		1,529.0
Deferred income taxes, net	0.0			0.0		0.0
Property and equipment, net	3.4			3.4		3.9
Other assets	122.6			122.6		125.8
Goodwill	0.0			0.0		0.0
Total assets	1,885.8			1,885.8		1,726.2
Liabilities [Abstract]
Accounts payable	0.0			0.0		0.0
Accrued expenses and other liabilities	90.7			90.7		86.8
Notes payable	1,093.6			1,093.6		1,132.6
Total liabilities	1,184.3			1,184.3		1,219.4
Revenues
Total revenues	188.7		156.6	601.1	364.0
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	127.0		93.9	360.4	257.7
Gain on sale of assets				0.0	0.0
Loss on extinguishment of debt	0.0			0.0
Other (income) expense	(8.6)		(6.1)	(21.4)	(17.7)
Income (loss) before income taxes	70.3		68.8	262.1	124.0
Depreciation and amortization				1.3	1.2
Cash provided by (used in) operating activities				96.2	(347.7)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	116.0			116.0		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	1,777.9			1,777.9		1,304.3
Land held for development	83.5			83.5		5.4
Land held for sale	0.0			0.0		0.0
Rental properties	0.0			0.0
Total inventories	1,861.4			1,861.4		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	2.2			2.2		1.1
Other assets	35.4			35.4		34.8
Goodwill	0.0			0.0		0.0
Total assets	2,015.0			2,015.0		1,739.9
Liabilities [Abstract]
Accounts payable	45.6			45.6		29.2
Accrued expenses and other liabilities	294.9			294.9		197.8
Notes payable	704.1			704.1		641.1
Total liabilities	1,044.6			1,044.6		868.1
Revenues
Total revenues	312.9		177.9	907.1	584.3
Inventory and land option charges	0.7		0.1	1.6	0.5
Cost of sales	257.1		157.1	753.8	510.3
Selling, General and Administrative Expense	16.9		11.2	48.7	32.8
Gain on sale of assets				0.0	(0.1)
Loss on extinguishment of debt	18.1			18.1
Other (income) expense	(0.3)		(0.7)	(1.4)	(4.8)
Income (loss) before income taxes	21.1		10.3	87.9	46.1
Depreciation and amortization				0.3	0.2
Cash provided by (used in) operating activities				(340.6)	(205.7)
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	51.8			51.8		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	436.3			436.3		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	81.1			81.1		308.9
Other assets	62.7			62.7		52.8
Goodwill	0.0			0.0		0.0
Total assets	632.2			632.2		379.4
Liabilities [Abstract]
Accounts payable	34.2			34.2		12.0
Accrued expenses and other liabilities	14.2			14.2		12.2
Notes payable	0.0			0.0		0.0
Total liabilities	48.4			48.4		24.2
Revenues
Total revenues	15.9		8.9	45.9	27.2
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	9.6		7.1	30.9	23.9
Gain on sale of assets				(0.9)	(59.4)
Loss on extinguishment of debt	0.0			0.0
Other (income) expense	(1.4)		2.4	5.8	5.8
Income (loss) before income taxes	7.7		(0.6)	10.1	56.9
Depreciation and amortization				10.5	5.8
Cash provided by (used in) operating activities				(58.2)	2.1
HomeBuildingMember
Assets [Abstract]
Cash and cash equivalents	1,673.8			1,673.8		2,551.1
Restricted cash	11.5			11.5		9.5
Inventories:
Construction in progress and finished homes	7,837.8			7,837.8		6,037.5
Residential land and lots — developed and under development	5,675.4			5,675.4		4,901.4
Land held for development	30.4			30.4		47.8
Land held for sale	23.5			23.5		28.3
Total inventories	13,870.2			13,870.2		11,015.0
Mortgage loans held for sale	0.0			0.0		0.0
Deferred income taxes, net	152.5			152.5		142.3
Property and equipment, net	280.3			280.3		372.8
Other assets	1,381.5			1,381.5		996.4
Goodwill	134.3			134.3		134.3
Total assets	17,504.1			17,504.1		15,221.4
Liabilities [Abstract]
Accounts payable	1,171.1			1,171.1		859.3
Accrued expenses and other liabilities	1,773.5			1,773.5		1,438.3
Notes payable	2,620.0			2,620.0		2,514.4
Total liabilities	5,564.6			5,564.6		4,812.0
Revenues
Total revenues	7,070.3		5,222.1	18,976.6	13,483.9
Inventory and land option charges	4.9		4.9	16.0	17.3
Cost of sales	5,238.5		4,097.4	14,253.2	10,621.4
Selling, General and Administrative Expense	502.0		415.1	1,422.7	1,135.3
Gain on sale of assets				(13.1)	0.0
Loss on extinguishment of debt	0.0			0.0
Other (income) expense	(7.1)		(0.2)	(10.4)	(9.7)
Income (loss) before income taxes	1,336.9		709.8	3,324.2	1,736.9
Depreciation and amortization				44.2	49.8
Cash provided by (used in) operating activities				276.1	1,157.0
East [Member] | HomeBuildingMember
Inventories:
Total inventories	1,605.6			1,605.6		1,328.3
Revenues
Total revenues	1,032.7		736.3	2,703.4	1,836.5
Inventory and land option charges	1.4		0.2	2.7	(0.1)
Income (loss) before income taxes	190.3		106.9	468.8	240.1
Midwest [Member] | HomeBuildingMember
Inventories:
Total inventories	1,225.9			1,225.9		958.5
Revenues
Total revenues	514.6		373.3	1,396.1	964.6
Inventory and land option charges	0.5		0.4	0.7	1.9
Income (loss) before income taxes	67.2		34.1	177.3	76.4
Southeast [Member] | HomeBuildingMember
Inventories:
Total inventories	3,510.6			3,510.6		2,919.9
Revenues
Total revenues	2,300.1		1,629.1	6,133.4	4,096.4
Inventory and land option charges	1.0		2.0	8.8	6.3
Income (loss) before income taxes	465.3		233.5	1,147.8	567.4
South Central [Member] | HomeBuildingMember
Inventories:
Total inventories	3,824.6			3,824.6		2,879.9
Revenues
Total revenues	1,709.8		1,376.4	4,688.8	3,401.5
Inventory and land option charges	1.5		1.9	2.1	6.3
Income (loss) before income taxes	323.1		201.8	854.6	490.7
Southwest [Member] | HomeBuildingMember
Inventories:
Total inventories	955.7			955.7		695.8
Revenues
Total revenues	291.0		216.1	766.7	626.8
Inventory and land option charges	0.0		0.1	0.4	0.1
Income (loss) before income taxes	46.7		29.7	124.8	95.0
West [Member] | HomeBuildingMember
Inventories:
Total inventories	2,536.6			2,536.6		2,009.1
Revenues
Total revenues	1,222.1		890.9	3,288.2	2,558.1
Inventory and land option charges	0.5		0.3	1.3	2.8
Income (loss) before income taxes	244.3		103.8	550.9	267.3
Corporate, Non-Segment [Member] | HomeBuildingMember
Inventories:
Total inventories	211.2			211.2		223.5
Multifamily [Member] | Other Segments [Member]
Inventories:
Rental properties	436.3			436.3
Total assets	458.3			458.3		$ 246.2
Land [Member]
Revenues
Home sales	55.8		25.8	154.6	112.6
Cost of Goods and Services Sold	27.8		14.2	85.4	67.1
Land [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	(303.2)		(175.5)	(865.8)	(548.6)
Cost of Goods and Services Sold	(243.0)		(153.0)	(707.7)	(477.6)
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	312.9		177.9	907.1	584.3
Cost of Goods and Services Sold	256.4		157.0	752.2	509.8
Land [Member] | Other Segments [Member]
Revenues
Home sales	15.9		8.9	45.9	27.2
Cost of Goods and Services Sold	0.0		0.0	0.0	0.0
Land [Member] | HomeBuildingMember
Revenues
Home sales	30.2		14.5	67.4	49.7
Cost of Goods and Services Sold	14.4		10.2	40.9	34.9
HomeBuildingMember
Revenues
Home sales	7,040.1		5,207.6	18,909.2	13,434.2
Cost of Goods and Services Sold	5,179.2		4,065.5	14,093.0	10,534.8
HomeBuildingMember | Intersegment Eliminations [Member]
Revenues
Home sales	0.0		0.0	0.0	0.0
Cost of Goods and Services Sold	(40.0)		(16.8)	(103.3)	(34.4)
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	7,040.1		5,207.6	18,909.2	13,434.2
Cost of Goods and Services Sold	5,219.2		4,082.3	14,196.3	10,569.2
Financial Services [Member]
Revenues
Home sales	188.7		156.6	601.1	364.0
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	188.7		156.6	601.1	364.0
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
Single Family [Member] | HomeBuildingMember
Inventories:
Rental properties	303.1			303.1
Revenues
Home sales	23.1	$ 31.8
Gain on sale of assets		$ (14.0)
Single Family [Member] | East [Member] | HomeBuildingMember
Inventories:
Rental properties	31.7			31.7
Single Family [Member] | Midwest [Member] | HomeBuildingMember
Inventories:
Rental properties	20.2			20.2
Single Family [Member] | Southeast [Member] | HomeBuildingMember
Inventories:
Rental properties	156.3			156.3
Single Family [Member] | South Central [Member] | HomeBuildingMember
Inventories:
Rental properties	33.5			33.5
Single Family [Member] | Southwest [Member] | HomeBuildingMember
Inventories:
Rental properties	25.5			25.5
Single Family [Member] | West [Member] | HomeBuildingMember
Inventories:
Rental properties	35.9			$ 35.9
MultifamilyMember | HomeBuildingMember
Revenues
Home sales	$ 128.5