Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Segment Reporting Information [Line Items]
Document Period End Date			Jun. 30, 2020
Assets [Abstract]
Cash and cash equivalents	$ 2,353.5		$ 2,353.5		$ 1,494.3
Restricted cash	19.2		19.2		19.7
Inventories:
Construction in progress and finished homes	5,847.3		5,847.3		5,245.0
Residential land and lots — developed and under development	6,198.8		6,198.8		5,939.4
Land held for development	64.5		64.5		77.8
Land held for sale	28.1		28.1		19.8
Total inventories	12,138.7		12,138.7		11,282.0
Mortgage loans held for sale	1,502.6		1,502.6		1,072.0
Deferred income taxes, net	158.6		158.6		163.1
Property and equipment, net	631.9		631.9		499.2
Other Assets	1,005.9		1,005.9		912.8
Goodwill	163.5		163.5		163.5
Total assets	17,973.9		17,973.9		15,606.6
Liabilities [Abstract]
Accounts payable	761.2		761.2		634.0
Accrued expenses and other liabilities	1,588.3		1,588.3		1,278.1
Notes payable	4,297.3		4,297.3		3,399.4
Total liabilities	6,646.8		6,646.8		5,311.5
Revenues
Total revenues	5,390.0	$ 4,906.3	13,910.8	$ 12,554.0
Inventory and land option charges	5.0	19.2	17.8	41.0
Cost of sales	4,084.7	3,831.6	10,619.7	9,839.4
Selling, General and Administrative Expense	527.5	480.0	1,450.1	1,327.0
Gain on sale of assets	0.0	(22.6)	(59.5)	(53.9)
Other (income) expense	(4.6)	(9.4)	(26.4)	(23.7)
Income (loss) before income taxes	782.4	626.7	1,926.9	1,465.2
Depreciation and amortization			57.4	52.4
Net cash provided by operating activities			588.9	80.7
Segment Reconciling Items [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	0.0		0.0		0.0
Residential land and lots — developed and under development	1.5		1.5		2.9
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Total inventories	1.5		1.5		2.9
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	(6.0)		(6.0)		(5.8)
Property and equipment, net	0.0		0.0		0.0
Other Assets	5.1		5.1		11.1
Goodwill	29.2		29.2		29.2
Total assets	29.8		29.8		37.4
Liabilities [Abstract]
Accounts payable	0.0		0.0		0.0
Accrued expenses and other liabilities	(18.2)		(18.2)		(12.6)
Notes payable	0.0		0.0		2.4
Total liabilities	(18.2)		(18.2)		(10.2)
Revenues
Total revenues	0.0	0.0	0.0	0.0
Inventory and land option charges	0.0	0.0	0.0	0.0
Cost of sales	(0.3)	8.2	(1.0)	20.0
Selling, General and Administrative Expense	0.2	0.1	0.4	0.3
Gain on sale of assets		1.5	0.0	2.4
Other (income) expense	0.0	0.0	0.0	0.0
Income (loss) before income taxes	0.1	(9.8)	0.6	(22.7)
Depreciation and amortization			0.4	0.4
Net cash provided by operating activities			0.0	(4.4)
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	(38.7)		(38.7)		(4.0)
Residential land and lots — developed and under development	(28.0)		(28.0)		(31.4)
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Total inventories	(66.7)		(66.7)		(35.4)
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	10.0		10.0		5.1
Property and equipment, net	(1.8)		(1.8)		0.0
Other Assets	(100.6)		(100.6)		(88.5)
Goodwill	0.0		0.0		0.0
Total assets	(159.1)		(159.1)		(118.8)
Liabilities [Abstract]
Accounts payable	0.0		0.0		0.0
Accrued expenses and other liabilities	(100.9)		(100.9)		(93.6)
Notes payable	(6.5)		(6.5)		0.0
Total liabilities	(107.4)		(107.4)		(93.6)
Revenues
Total revenues	(175.5)	(73.2)	(548.6)	(141.8)
Inventory and land option charges	0.0	0.0	0.0	0.0
Cost of sales	(169.5)	(67.3)	(511.0)	(125.3)
Selling, General and Administrative Expense	0.0	0.0	0.0	0.0
Gain on sale of assets		0.0	0.0	0.0
Other (income) expense	0.0	0.0	0.0	0.0
Income (loss) before income taxes	(6.0)	(5.9)	(37.6)	(16.5)
Depreciation and amortization			0.0	0.0
Net cash provided by operating activities			(16.8)	(2.5)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	75.1		75.1		43.4
Restricted cash	10.0		10.0		11.6
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	1,502.6		1,502.6		1,072.0
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	3.7		3.7		3.2
Other Assets	128.9		128.9		68.3
Goodwill	0.0		0.0		0.0
Total assets	1,720.3		1,720.3		1,198.5
Liabilities [Abstract]
Accounts payable	0.1		0.1		7.0
Accrued expenses and other liabilities	103.7		103.7		53.0
Notes payable	1,172.9		1,172.9		888.9
Total liabilities	1,276.7		1,276.7		948.9
Revenues
Total revenues	156.6	119.6	364.0	306.4
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	93.9	76.4	257.7	213.4
Gain on sale of assets		0.0	0.0	0.0
Other (income) expense	(6.1)	(4.9)	(17.7)	(12.6)
Income (loss) before income taxes	68.8	48.1	124.0	105.6
Depreciation and amortization			1.2	1.1
Net cash provided by operating activities			(347.7)	(65.5)
Home Building [Member]
Assets [Abstract]
Cash and cash equivalents	1,896.8		1,896.8		1,043.0
Restricted cash	9.0		9.0		8.0
Inventories:
Construction in progress and finished homes	5,886.0		5,886.0		5,249.0
Residential land and lots — developed and under development	4,953.6		4,953.6		4,956.1
Land held for development	49.1		49.1		60.7
Land held for sale	28.1		28.1		19.8
Total inventories	10,916.8		10,916.8		10,285.6
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	154.6		154.6		146.4
Property and equipment, net	344.0		344.0		272.4
Other Assets	880.7		880.7		826.2
Goodwill	134.3		134.3		134.3
Total assets	14,336.2		14,336.2		12,715.9
Liabilities [Abstract]
Accounts payable	720.7		720.7		598.6
Accrued expenses and other liabilities	1,423.1		1,423.1		1,152.5
Notes payable	2,490.3		2,490.3		2,047.6
Total liabilities	4,634.1		4,634.1		3,798.7
Revenues
Total revenues	5,222.1	4,762.1	13,483.9	12,175.0
Inventory and land option charges	4.9	19.2	17.3	41.0
Cost of sales	4,097.4	3,815.4	10,621.4	9,795.1
Selling, General and Administrative Expense	415.1	387.4	1,135.3	1,071.4
Gain on sale of assets		0.0	0.0	(2.0)
Other (income) expense	(0.2)	(2.5)	(9.7)	(6.1)
Income (loss) before income taxes	709.8	561.8	1,736.9	1,316.6
Depreciation and amortization			49.8	46.4
Net cash provided by operating activities			1,157.0	605.7
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	355.6		355.6		382.8
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	1,271.7		1,271.7		1,011.8
Land held for development	15.4		15.4		17.1
Land held for sale	0.0		0.0		0.0
Total inventories	1,287.1		1,287.1		1,028.9
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	0.0		0.0		17.4
Property and equipment, net	1.2		1.2		2.4
Other Assets	41.5		41.5		24.2
Goodwill	0.0		0.0		0.0
Total assets	1,685.4		1,685.4		1,455.7
Liabilities [Abstract]
Accounts payable	26.0		26.0		16.8
Accrued expenses and other liabilities	171.9		171.9		169.5
Notes payable	640.6		640.6		460.5
Total liabilities	838.5		838.5		646.8
Revenues
Total revenues	177.9	88.2	584.3	192.0
Inventory and land option charges	0.1	0.0	0.5	0.0
Cost of sales	157.1	75.3	510.3	149.6
Selling, General and Administrative Expense	11.2	7.9	32.8	19.8
Gain on sale of assets		(1.5)	(0.1)	(2.4)
Other (income) expense	(0.7)	(1.9)	(4.8)	(4.6)
Income (loss) before income taxes	10.3	8.4	46.1	29.6
Depreciation and amortization			0.2	0.2
Net cash provided by operating activities			(205.7)	(450.1)
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	26.0		26.0		25.1
Restricted cash	0.2		0.2		0.1
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	284.8		284.8		221.2
Other Assets	50.3		50.3		71.5
Goodwill	0.0		0.0		0.0
Total assets	361.3		361.3		317.9
Liabilities [Abstract]
Accounts payable	14.4		14.4		11.6
Accrued expenses and other liabilities	8.7		8.7		9.3
Notes payable	0.0		0.0		0.0
Total liabilities	23.1		23.1		20.9
Revenues
Total revenues	8.9	9.6	27.2	22.4
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	7.1	8.2	23.9	22.1
Gain on sale of assets		(22.6)	(59.4)	(51.9)
Other (income) expense	2.4	(0.1)	5.8	(0.4)
Income (loss) before income taxes	(0.6)	24.1	56.9	52.6
Depreciation and amortization			5.8	4.3
Net cash provided by operating activities			2.1	(2.5)
East [Member] | Home Building [Member]
Inventories:
Total inventories	1,262.3		1,262.3		1,288.8
Revenues
Total revenues	736.3	675.2	1,836.5	1,640.9
Inventory and land option charges	0.2	0.6	(0.1)	2.3
Income (loss) before income taxes	106.9	74.1	240.1	158.0
Midwest [Member] | Home Building [Member]
Inventories:
Total inventories	917.8		917.8		836.8
Revenues
Total revenues	373.3	303.3	964.6	800.5
Inventory and land option charges	0.4	1.3	1.9	1.8
Income (loss) before income taxes	34.1	18.7	76.4	38.8
Southeast [Member] | Home Building [Member]
Inventories:
Total inventories	2,805.1		2,805.1		2,768.0
Revenues
Total revenues	1,629.1	1,388.1	4,096.4	3,607.2
Inventory and land option charges	2.0	5.1	6.3	8.6
Income (loss) before income taxes	233.5	168.4	567.4	411.6
South Central [Member] | Home Building [Member]
Inventories:
Total inventories	2,903.2		2,903.2		2,533.2
Revenues
Total revenues	1,376.4	1,178.0	3,401.5	3,040.8
Inventory and land option charges	1.9	3.0	6.3	4.9
Income (loss) before income taxes	201.8	164.3	490.7	389.6
Southwest [Member] | Home Building [Member]
Inventories:
Total inventories	669.6		669.6		574.4
Revenues
Total revenues	216.1	240.6	626.8	557.5
Inventory and land option charges	0.1	0.3	0.1	0.5
Income (loss) before income taxes	29.7	33.5	95.0	69.8
West [Member] | Home Building [Member]
Inventories:
Total inventories	2,129.0		2,129.0		2,056.0
Revenues
Total revenues	890.9	976.9	2,558.1	2,528.1
Inventory and land option charges	0.3	8.9	2.8	22.9
Income (loss) before income taxes	103.8	102.8	267.3	248.8
Corporate, Non-Segment [Member] | Home Building [Member]
Inventories:
Total inventories	229.8		229.8		228.4
Multifamily [Member] | Other Segments [Member]
Inventories:
Total assets	224.8		224.8		$ 204.0
Financial Service [Member]
Revenues
Home sales	156.6	119.6	364.0	306.4
Financial Service [Member] | Segment Reconciling Items [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Service [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Service [Member] | Financial Services [Member]
Revenues
Home sales	156.6	119.6	364.0	306.4
Financial Service [Member] | Home Building [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Service [Member] | Forestar Group [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Financial Service [Member] | Other Segments [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Land [Member]
Revenues
Home sales	25.8	52.1	112.6	121.8
Cost of Goods and Services Sold	14.2	40.7	67.1	85.0
Land [Member] | Segment Reconciling Items [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	(0.3)	8.2	(1.0)	20.0
Land [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	(175.5)	(73.2)	(548.6)	(141.8)
Cost of Goods and Services Sold	(152.7)	(66.0)	(476.6)	(122.2)
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] | Home Building [Member]
Revenues
Home sales	14.5	27.5	49.7	49.2
Cost of Goods and Services Sold	10.2	23.2	34.9	37.6
Land [Member] | Forestar Group [Member]
Revenues
Home sales	177.9	88.2	584.3	192.0
Cost of Goods and Services Sold	157.0	75.3	509.8	149.6
Land [Member] | Other Segments [Member]
Revenues
Home sales	8.9	9.6	27.2	22.4
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Home Building [Member]
Revenues
Home sales	5,207.6	4,734.6	13,434.2	12,125.8
Cost of Goods and Services Sold	4,065.5	3,771.7	10,534.8	9,713.4
Home Building [Member] | Segment Reconciling Items [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
Home Building [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0	0.0	0.0
Cost of Goods and Services Sold	(16.8)	(1.3)	(34.4)	(3.1)
Home Building [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
Home Building [Member] | Home Building [Member]
Revenues
Home sales	5,207.6	4,734.6	13,434.2	12,125.8
Cost of Goods and Services Sold	4,082.3	3,773.0	10,569.2	9,716.5
Home Building [Member] | 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
Home Building [Member] | 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