Segment Information (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Segment Reporting Information [Line Items]
Document Period End Date				Mar. 31, 2020
Assets [Abstract]
Cash and cash equivalents	$ 1,522.8			$ 1,522.8		$ 1,494.3
Restricted cash	14.5			14.5		19.7
Inventories:
Construction in progress and finished homes	5,969.4			5,969.4		5,245.0
Residential land and lots — developed and under development	6,151.9			6,151.9		5,939.4
Land held for development	77.1			77.1		77.8
Land held for sale	26.1			26.1		19.8
Total inventories	12,224.5			12,224.5		11,282.0
Mortgage loans held for sale	1,379.4			1,379.4		1,072.0
Deferred income taxes, net	149.5			149.5		163.1
Property and equipment, net	589.7			589.7		499.2
Other Assets	1,086.1			1,086.1		912.8
Goodwill	163.5			163.5		163.5
Total assets	17,130.0			17,130.0		15,606.6
Liabilities [Abstract]
Accounts payable	707.0			707.0		634.0
Accrued expenses and other liabilities	1,381.6			1,381.6		1,278.1
Notes payable	4,306.6			4,306.6		3,399.4
Total liabilities	6,395.2			6,395.2		5,311.5
Revenues
Total revenues	4,500.0		$ 4,128.7	8,520.7	$ 7,647.7
Inventory and land option charges	8.9		13.8	12.8	21.8
Cost of sales	3,450.8		3,256.7	6,535.0	6,007.8
Selling, General and Administrative Expense	466.8		444.2	922.6	847.0
Gain on sale of assets	(28.5)		(29.3)	(59.5)	(31.3)
Other (income) expense	(10.4)		(5.7)	(21.9)	(14.3)
Income (loss) before income taxes	621.3		462.8	1,144.5	838.5
Depreciation and amortization				38.8	34.1
Net cash used in operating activities				(395.1)	(461.7)
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	1.2			1.2		2.9
Land held for development	0.0			0.0		0.0
Land held for sale	0.0			0.0		0.0
Total inventories	1.2			1.2		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	11.6			11.6		11.1
Goodwill	29.2			29.2		29.2
Total assets	36.0			36.0		37.4
Liabilities [Abstract]
Accounts payable	0.0			0.0		0.0
Accrued expenses and other liabilities	(11.8)			(11.8)		(12.6)
Notes payable	0.0			0.0		2.4
Total liabilities	(11.8)			(11.8)		(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	0.0		8.2	(0.7)	11.8
Selling, General and Administrative Expense	0.1		0.1	0.2	0.3
Gain on sale of assets	0.0		0.0	0.0	0.9
Other (income) expense	0.0		0.0	0.0	0.0
Income (loss) before income taxes	(0.1)		(8.3)	0.5	(13.0)
Depreciation and amortization				0.3	0.3
Net cash used in 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	(34.4)			(34.4)		(4.0)
Residential land and lots — developed and under development	(29.9)			(29.9)		(31.4)
Land held for development	0.0			0.0		0.0
Land held for sale	0.0			0.0		0.0
Total inventories	(64.3)			(64.3)		(35.4)
Mortgage loans held for sale	0.0			0.0		0.0
Deferred income taxes, net	10.8			10.8		5.1
Property and equipment, net	(1.0)			(1.0)		0.0
Other Assets	(97.6)			(97.6)		(88.5)
Goodwill	0.0			0.0		0.0
Total assets	(152.1)			(152.1)		(118.8)
Liabilities [Abstract]
Accounts payable	0.0			0.0		0.0
Accrued expenses and other liabilities	(105.1)			(105.1)		(93.6)
Notes payable	0.0			0.0		0.0
Total liabilities	(105.1)			(105.1)		(93.6)
Revenues
Total revenues	(151.9)		(39.7)	(373.2)	(68.7)
Inventory and land option charges	0.0		0.0	0.0	0.0
Cost of sales	(141.4)		(32.5)	(341.6)	(58.0)
Selling, General and Administrative Expense	0.0		0.0	0.0	0.0
Gain on sale of assets	0.0		0.0	0.0	0.0
Other (income) expense	0.0		0.0	0.0	0.0
Income (loss) before income taxes	(10.5)		(7.2)	(31.6)	(10.7)
Depreciation and amortization				0.0	0.0
Net cash used in operating activities				(16.8)	(2.7)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	45.8			45.8		43.4
Restricted cash	6.6			6.6		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	1,379.4			1,379.4		1,072.0
Deferred income taxes, net	0.0			0.0		0.0
Property and equipment, net	3.3			3.3		3.2
Other Assets	203.5			203.5		68.3
Goodwill	0.0			0.0		0.0
Total assets	1,638.6			1,638.6		1,198.5
Liabilities [Abstract]
Accounts payable	0.2			0.2		7.0
Accrued expenses and other liabilities	146.7			146.7		53.0
Notes payable	1,186.5			1,186.5		888.9
Total liabilities	1,333.4			1,333.4		948.9
Revenues
Total revenues	104.5		101.6	207.4	186.9
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	85.9		71.3	163.8	137.0
Gain on sale of assets	0.0		0.0	0.0	0.0
Other (income) expense	(6.1)		(3.7)	(11.6)	(7.7)
Income (loss) before income taxes	24.7		34.0	55.2	57.6
Depreciation and amortization				0.8	0.7
Net cash used in operating activities				(312.1)	48.8
Home Building [Member]
Assets [Abstract]
Cash and cash equivalents	1,012.6			1,012.6		1,043.0
Restricted cash	7.8			7.8		8.0
Inventories:
Construction in progress and finished homes	6,003.8			6,003.8		5,249.0
Residential land and lots — developed and under development	5,009.6			5,009.6		4,956.1
Land held for development	48.9			48.9		60.7
Land held for sale	26.1			26.1		19.8
Total inventories	11,088.4			11,088.4		10,285.6
Mortgage loans held for sale	0.0			0.0		0.0
Deferred income taxes, net	136.7			136.7		146.4
Property and equipment, net	327.2			327.2		272.4
Other Assets	891.5			891.5		826.2
Goodwill	134.3			134.3		134.3
Total assets	13,598.5			13,598.5		12,715.9
Liabilities [Abstract]
Accounts payable	670.6			670.6		598.6
Accrued expenses and other liabilities	1,165.7			1,165.7		1,152.5
Notes payable	2,480.0			2,480.0		2,047.6
Total liabilities	4,316.3			4,316.3		3,798.7
Revenues
Total revenues	4,378.8		3,995.4	8,261.8	7,412.9
Inventory and land option charges	8.8		13.8	12.4	21.8
Cost of sales	3,455.6		3,237.3	6,524.1	5,979.7
Selling, General and Administrative Expense	361.8		359.3	720.2	683.9
Gain on sale of assets	0.0		0.0	0.0	(2.0)
Other (income) expense	(4.1)		(1.6)	(9.6)	(3.5)
Income (loss) before income taxes	565.5		400.4	1,027.1	754.8
Depreciation and amortization				33.7	30.1
Net cash used in operating activities				52.1	(215.9)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	438.2			438.2		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	1,171.0			1,171.0		1,011.8
Land held for development	28.2			28.2		17.1
Land held for sale	0.0			0.0		0.0
Total inventories	1,199.2			1,199.2		1,028.9
Mortgage loans held for sale	0.0			0.0		0.0
Deferred income taxes, net	8.0			8.0		17.4
Property and equipment, net	1.0			1.0		2.4
Other Assets	29.2			29.2		24.2
Goodwill	0.0			0.0		0.0
Total assets	1,675.6			1,675.6		1,455.7
Liabilities [Abstract]
Accounts payable	23.1			23.1		16.8
Accrued expenses and other liabilities	175.6			175.6		169.5
Notes payable	640.1			640.1		460.5
Total liabilities	838.8			838.8		646.8
Revenues
Total revenues	159.1		65.4	406.4	103.8
Inventory and land option charges	0.1		0.0	0.4	0.0
Cost of sales	136.6		43.7	353.2	74.3
Selling, General and Administrative Expense	11.2		6.2	21.7	11.9
Gain on sale of assets	(0.3)		0.0	(0.1)	(0.9)
Other (income) expense	(2.1)		(0.9)	(4.2)	(2.8)
Income (loss) before income taxes	13.7		16.4	35.8	21.3
Depreciation and amortization				0.1	0.1
Net cash used in operating activities				(123.8)	(283.4)
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	26.2			26.2		25.1
Restricted cash	0.1			0.1		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	259.2			259.2		221.2
Other Assets	47.9			47.9		71.5
Goodwill	0.0			0.0		0.0
Total assets	333.4			333.4		317.9
Liabilities [Abstract]
Accounts payable	13.1			13.1		11.6
Accrued expenses and other liabilities	10.5			10.5		9.3
Notes payable	0.0			0.0		0.0
Total liabilities	23.6			23.6		20.9
Revenues
Total revenues	9.5		6.0	18.3	12.8
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	7.8		7.3	16.7	13.9
Gain on sale of assets	(28.2)		(29.3)	(59.4)	(29.3)
Other (income) expense	1.9		0.5	3.5	(0.3)
Income (loss) before income taxes	28.0		27.5	57.5	28.5
Depreciation and amortization				3.9	2.9
Net cash used in operating activities				5.5	(4.1)
East [Member] | Home Building [Member]
Inventories:
Total inventories	1,329.8			1,329.8		1,288.8
Revenues
Total revenues	579.7		518.2	1,100.2	965.7
Inventory and land option charges	(0.5)		0.3	(0.3)	1.7
Income (loss) before income taxes	73.3		45.9	133.2	83.9
Midwest [Member] | Home Building [Member]
Inventories:
Total inventories	903.2			903.2		836.8
Revenues
Total revenues	308.7		248.1	591.3	497.2
Inventory and land option charges	1.4		0.2	1.5	0.5
Income (loss) before income taxes	23.6		9.5	42.3	20.2
Southeast [Member] | Home Building [Member]
Inventories:
Total inventories	2,968.1			2,968.1		2,768.0
Revenues
Total revenues	1,316.7		1,205.3	2,467.3	2,219.2
Inventory and land option charges	2.8		2.2	4.2	3.5
Income (loss) before income taxes	187.8		131.0	333.9	243.2
South Central [Member] | Home Building [Member]
Inventories:
Total inventories	2,845.4			2,845.4		2,533.2
Revenues
Total revenues	1,066.4		990.2	2,025.1	1,862.7
Inventory and land option charges	2.9		1.4	4.4	1.9
Income (loss) before income taxes	156.2		119.3	288.9	225.3
Southwest [Member] | Home Building [Member]
Inventories:
Total inventories	633.8			633.8		574.4
Revenues
Total revenues	199.7		173.2	410.7	316.9
Inventory and land option charges	0.0		0.1	0.1	0.2
Income (loss) before income taxes	30.8		18.6	65.3	36.3
West [Member] | Home Building [Member]
Inventories:
Total inventories	2,174.1			2,174.1		2,056.0
Revenues
Total revenues	907.6		860.4	1,667.2	1,551.2
Inventory and land option charges	2.2		9.6	2.5	14.0
Income (loss) before income taxes	93.8		76.1	163.5	145.9
Corporate, Non-Segment [Member] | Home Building [Member]
Inventories:
Total inventories	234.0			234.0		228.4
Multifamily [Member] | Other Segments [Member]
Inventories:
Total assets	202.8			202.8		$ 204.0
Revenues
Gain on sale of assets	(28.2)	$ (31.2)
Financial Service [Member]
Revenues
Home sales	104.5		101.6	207.4	186.9
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	104.5		101.6	207.4	186.9
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	32.2		46.6	86.7	69.6
Cost of Goods and Services Sold	17.4		29.4	52.9	44.3
Land [Member] | Segment Reconciling Items [Member]
Revenues
Home sales	0.0		0.0	0.0	0.0
Cost of Goods and Services Sold	0.0		8.2	(0.7)	11.8
Land [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	(151.9)		(39.7)	(373.2)	(68.7)
Cost of Goods and Services Sold	(130.4)		(31.8)	(323.9)	(56.3)
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	15.5		14.9	35.2	21.7
Cost of Goods and Services Sold	11.3		9.3	24.7	14.5
Land [Member] | Forestar Group [Member]
Revenues
Home sales	159.1		65.4	406.4	103.8
Cost of Goods and Services Sold	136.5		43.7	352.8	74.3
Land [Member] | Other Segments [Member]
Revenues
Home sales	9.5		6.0	18.3	12.8
Cost of Goods and Services Sold	0.0		0.0	0.0	0.0
Home Building [Member]
Revenues
Home sales	4,363.3		3,980.5	8,226.6	7,391.2
Cost of Goods and Services Sold	3,424.5		3,213.5	6,469.3	5,941.7
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	(11.0)		(0.7)	(17.7)	(1.7)
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	4,363.3		3,980.5	8,226.6	7,391.2
Cost of Goods and Services Sold	3,435.5		3,214.2	6,487.0	5,943.4
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