Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Assets [Abstract]
Cash and cash equivalents	$ 2,454.9		$ 3,018.5
Restricted cash	20.0		21.6
Inventories:
Construction in progress and finished homes	6,598.8		5,984.1
Residential land and lots — developed and under development	6,856.1		6,171.8
Land held for development	97.8		53.2
Land held for sale	23.8		28.3
Total inventories	13,576.5		12,237.4
Mortgage loans held for sale	1,438.9		1,529.0
Deferred income taxes, net	142.0		144.9
Property and equipment, net	748.4		683.7
Other assets	1,236.2		1,113.7
Goodwill	163.5		163.5
Total assets	19,780.4		18,912.3
Liabilities [Abstract]
Accounts payable	845.6		900.5
Accrued expenses and other liabilities	1,938.9		1,607.0
Notes payable	4,225.6		4,283.3
Total liabilities	7,010.1		6,790.8
Revenues
Total revenues	5,933.4	$ 4,020.7
Inventory and land option charges	8.3	3.8
Cost of sales	4,332.5	3,084.2
Selling, General and Administrative Expense	585.9	455.8
Gain on sale of assets	(14.0)	(31.1)
Other (income) expense	(5.3)	(11.5)
Income (loss) before income taxes	1,034.3	523.3
Depreciation and amortization	20.4	19.0
Net cash used in operating activities	(252.1)	(113.8)
Intersegment Eliminations [Member]
Assets [Abstract]
Cash and cash equivalents	0.0		0.0
Restricted cash	0.0		0.0
Inventories:
Construction in progress and finished homes	(66.3)		(53.4)
Residential land and lots — developed and under development	(36.1)		(33.9)
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	(102.4)		(87.3)
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	(0.6)		2.6
Property and equipment, net	(4.6)		(3.0)
Other assets	(92.9)		(96.1)
Goodwill	29.2		29.2
Total assets	(171.3)		(154.6)
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	(133.4)		(128.1)
Notes payable	(3.8)		(4.8)
Total liabilities	(137.2)		(132.9)
Revenues
Total revenues	(294.2)	(221.2)
Inventory and land option charges	0.0	0.0
Cost of sales	(277.2)	(200.8)
Selling, General and Administrative Expense	0.1	0.2
Gain on sale of assets	0.0	0.0
Other (income) expense	(0.7)	0.0
Income (loss) before income taxes	(16.4)	(20.6)
Depreciation and amortization	0.1	0.1
Net cash used in operating activities	1.5	(11.1)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	65.3		55.6
Restricted cash	9.0		11.9
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
Total inventories	0.0		0.0
Mortgage loans held for sale	1,438.9		1,529.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	3.7		3.9
Other assets	121.5		125.8
Goodwill	0.0		0.0
Total assets	1,638.4		1,726.2
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	101.0		86.8
Notes payable	969.1		1,132.6
Total liabilities	1,070.1		1,219.4
Revenues
Total revenues	187.2	102.9
Inventory and land option charges	0.0	0.0
Cost of sales	0.0	0.0
Selling, General and Administrative Expense	109.5	77.9
Gain on sale of assets	0.0	0.0
Other (income) expense	(6.4)	(5.5)
Income (loss) before income taxes	84.1	30.5
Depreciation and amortization	0.4	0.4
Net cash used in operating activities	173.1	83.5
Home Building [Member]
Assets [Abstract]
Cash and cash equivalents	2,129.3		2,551.1
Restricted cash	10.8		9.5
Inventories:
Construction in progress and finished homes	6,665.1		6,037.5
Residential land and lots — developed and under development	5,407.3		4,901.4
Land held for development	42.3		47.8
Land held for sale	23.8		28.3
Total inventories	12,138.5		11,015.0
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	142.6		142.3
Property and equipment, net	388.3		372.8
Other assets	1,121.7		996.4
Goodwill	134.3		134.3
Total assets	16,065.5		15,221.4
Liabilities [Abstract]
Accounts payable	786.4		859.3
Accrued expenses and other liabilities	1,741.6		1,438.3
Notes payable	2,606.2		2,514.4
Total liabilities	5,134.2		4,812.0
Revenues
Total revenues	5,718.6	3,883.0
Inventory and land option charges	7.9	3.5
Cost of sales	4,346.8	3,068.4
Selling, General and Administrative Expense	451.2	358.4
Gain on sale of assets	(13.1)	0.0
Other (income) expense	(1.5)	(5.4)
Income (loss) before income taxes	935.2	461.6
Depreciation and amortization	15.4	16.5
Net cash used in operating activities	(269.2)	(178.4)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	237.4		394.3
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	1,484.9		1,304.3
Land held for development	55.5		5.4
Land held for sale	0.0		0.0
Total inventories	1,540.4		1,309.7
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	1.9		1.1
Other assets	29.4		34.8
Goodwill	0.0		0.0
Total assets	1,809.1		1,739.9
Liabilities [Abstract]
Accounts payable	42.2		29.2
Accrued expenses and other liabilities	218.8		197.8
Notes payable	654.1		641.1
Total liabilities	915.1		868.1
Revenues
Total revenues	307.1	247.2
Inventory and land option charges	0.4	0.3
Cost of sales	262.9	216.6
Selling, General and Administrative Expense	15.5	10.5
Gain on sale of assets	0.0	0.1
Other (income) expense	(0.5)	(2.2)
Income (loss) before income taxes	29.2	22.2
Depreciation and amortization	0.1	0.1
Net cash used in operating activities	(158.7)	(11.7)
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	22.9		17.5
Restricted cash	0.2		0.2
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
Total inventories	0.0		0.0
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	359.1		308.9
Other assets	56.5		52.8
Goodwill	0.0		0.0
Total assets	438.7		379.4
Liabilities [Abstract]
Accounts payable	17.0		12.0
Accrued expenses and other liabilities	10.9		12.2
Notes payable	0.0		0.0
Total liabilities	27.9		24.2
Revenues
Total revenues	14.7	8.8
Inventory and land option charges	0.0	0.0
Cost of sales	0.0	0.0
Selling, General and Administrative Expense	9.6	8.8
Gain on sale of assets	(0.9)	(31.2)
Other (income) expense	3.8	1.6
Income (loss) before income taxes	2.2	29.6
Depreciation and amortization	4.4	1.9
Net cash used in operating activities	1.2	3.9
East [Member] | Home Building [Member]
Inventories:
Total inventories	1,444.3		1,328.3
Revenues
Total revenues	809.2	520.5
Inventory and land option charges	0.6	0.1
Income (loss) before income taxes	132.5	59.9
Midwest [Member] | Home Building [Member]
Inventories:
Total inventories	1,079.7		958.5
Revenues
Total revenues	413.1	282.6
Inventory and land option charges	0.1	0.1
Income (loss) before income taxes	49.2	18.6
Southeast [Member] | Home Building [Member]
Inventories:
Total inventories	3,142.8		2,919.9
Revenues
Total revenues	1,778.4	1,150.6
Inventory and land option charges	6.7	1.4
Income (loss) before income taxes	305.8	146.3
South Central [Member] | Home Building [Member]
Inventories:
Total inventories	3,255.8		2,879.9
Revenues
Total revenues	1,474.9	958.7
Inventory and land option charges	0.1	1.5
Income (loss) before income taxes	264.7	132.6
Southwest [Member] | Home Building [Member]
Inventories:
Total inventories	849.2		695.8
Revenues
Total revenues	232.6	211.0
Inventory and land option charges	0.1	0.1
Income (loss) before income taxes	38.6	34.5
West [Member] | Home Building [Member]
Inventories:
Total inventories	2,143.6		2,009.1
Revenues
Total revenues	1,010.4	759.6
Inventory and land option charges	0.3	0.3
Income (loss) before income taxes	144.4	69.7
Corporate, Non-Segment [Member] | Home Building [Member]
Inventories:
Total inventories	223.1		223.5
Multifamily [Member] | Other Segments [Member]
Inventories:
Total assets	294.3		$ 246.2
Financial Service [Member]
Revenues
Home sales	187.2	102.9
Financial Service [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0
Financial Service [Member] | Financial Services [Member]
Revenues
Home sales	187.2	102.9
Financial Service [Member] | Home Building [Member]
Revenues
Home sales	0.0	0.0
Financial Service [Member] | Forestar Group [Member]
Revenues
Home sales	0.0	0.0
Financial Service [Member] | Other Segments [Member]
Revenues
Home sales	0.0	0.0
Land [Member]
Revenues
Home sales	47.5	54.5
Cost of Goods and Services Sold	26.8	35.5
Land [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	(294.2)	(221.2)
Cost of Goods and Services Sold	(249.5)	(194.1)
Land [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Land [Member] | Home Building [Member]
Revenues
Home sales	19.9	19.7
Cost of Goods and Services Sold	13.8	13.3
Land [Member] | Forestar Group [Member]
Revenues
Home sales	307.1	247.2
Cost of Goods and Services Sold	262.5	216.3
Land [Member] | Other Segments [Member]
Revenues
Home sales	14.7	8.8
Cost of Goods and Services Sold	0.0	0.0
Home Building [Member]
Revenues
Home sales	5,698.7	3,863.3
Cost of Goods and Services Sold	4,297.4	3,044.9
Home Building [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(27.7)	(6.7)
Home Building [Member] | Financial Services [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Home Building [Member] | Home Building [Member]
Revenues
Home sales	5,698.7	3,863.3
Cost of Goods and Services Sold	4,325.1	3,051.6
Home Building [Member] | Forestar Group [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Home Building [Member] | Other Segments [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	$ 0.0	$ 0.0