Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019
Segment Reporting Information [Line Items]
Document Period End Date	Dec. 31, 2019
Assets [Abstract]
Cash and cash equivalents	$ 1,583.3		$ 1,494.3
Restricted cash	12.6		19.7
Inventories:
Construction in progress and finished homes	5,580.1		5,245.0
Residential land and lots — developed and under development	6,214.1		5,939.4
Land held for development	82.1		77.8
Land held for sale	22.9		19.8
Total inventories	11,899.2		11,282.0
Mortgage loans held for sale	1,025.1		1,072.0
Deferred income taxes, net	154.1		163.1
Property and equipment, net	563.4		499.2
Other Assets	921.7		912.8
Goodwill	163.5		163.5
Total assets	16,322.9		15,606.6
Liabilities [Abstract]
Accounts payable	659.3		634.0
Accrued expenses and other liabilities	1,377.4		1,278.1
Notes payable	3,783.3		3,399.4
Total liabilities	5,820.0		5,311.5
Revenues
Total revenues	4,020.7	$ 3,519.0
Inventory and land option charges	3.8	8.0
Cost of sales	3,084.2	2,751.1
Selling, General and Administrative Expense	455.8	402.8
Gain on sale of assets	(31.1)	(2.0)
Other (income) expense	(11.5)	(8.6)
Income (loss) before income taxes	523.3	375.7
Depreciation and amortization	19.0	16.9
Net cash (used in) provided by operating activities	(113.8)	(373.1)
Segment Reconciling Items [Member]
Assets [Abstract]
Cash and cash equivalents	0.0		0.0
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	2.1		2.9
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	2.1		2.9
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	(6.0)		(5.8)
Property and equipment, net	0.0		0.0
Other Assets	10.5		11.1
Goodwill	29.2		29.2
Total assets	35.8		37.4
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	(13.1)		(12.6)
Notes payable	1.0		2.4
Total liabilities	(12.1)		(10.2)
Revenues
Total revenues	0.0	0.0
Inventory and land option charges	0.0	0.0
Cost of sales	(0.7)	3.6
Selling, General and Administrative Expense	0.2	0.1
Gain on sale of assets	0.0	0.9
Other (income) expense	0.0	0.0
Income (loss) before income taxes	0.5	(4.6)
Depreciation and amortization	0.1	0.1
Net cash (used in) provided by operating activities	0.0	(4.4)
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	(23.2)		(4.0)
Residential land and lots — developed and under development	(32.1)		(31.4)
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	(55.3)		(35.4)
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	8.2		5.1
Property and equipment, net	(1.0)		0.0
Other Assets	(86.1)		(88.5)
Goodwill	0.0		0.0
Total assets	(134.2)		(118.8)
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	(94.5)		(93.6)
Notes payable	0.0		0.0
Total liabilities	(94.5)		(93.6)
Revenues
Total revenues	(221.2)	(29.0)
Inventory and land option charges	0.0	0.0
Cost of sales	(200.1)	(25.5)
Selling, General and Administrative Expense	0.0	0.0
Gain on sale of assets	0.0	0.0
Other (income) expense	0.0	0.0
Income (loss) before income taxes	(21.1)	(3.5)
Depreciation and amortization	0.0	0.0
Net cash (used in) provided by operating activities	(11.1)	0.0
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	37.5		43.4
Restricted cash	5.9		11.6
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	1,025.1		1,072.0
Deferred income taxes, net	0.0		0.0
Property and equipment, net	3.1		3.2
Other Assets	59.7		68.3
Goodwill	0.0		0.0
Total assets	1,131.3		1,198.5
Liabilities [Abstract]
Accounts payable	1.4		7.0
Accrued expenses and other liabilities	62.3		53.0
Notes payable	850.2		888.9
Total liabilities	913.9		948.9
Revenues
Total revenues	102.9	85.3
Inventory and land option charges	0.0	0.0
Cost of sales	0.0	0.0
Selling, General and Administrative Expense	77.9	65.6
Gain on sale of assets	0.0	0.0
Other (income) expense	(5.5)	(3.9)
Income (loss) before income taxes	30.5	23.6
Depreciation and amortization	0.4	0.4
Net cash (used in) provided by operating activities	83.5	193.8
Home Building [Member]
Assets [Abstract]
Cash and cash equivalents	1,152.9		1,043.0
Restricted cash	6.6		8.0
Inventories:
Construction in progress and finished homes	5,603.3		5,249.0
Residential land and lots — developed and under development	5,192.7		4,956.1
Land held for development	66.7		60.7
Land held for sale	22.9		19.8
Total inventories	10,885.6		10,285.6
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	140.1		146.4
Property and equipment, net	292.8		272.4
Other Assets	864.4		826.2
Goodwill	134.3		134.3
Total assets	13,476.7		12,715.9
Liabilities [Abstract]
Accounts payable	624.7		598.6
Accrued expenses and other liabilities	1,240.6		1,152.5
Notes payable	2,470.0		2,047.6
Total liabilities	4,335.3		3,798.7
Revenues
Total revenues	3,883.0	3,417.3
Inventory and land option charges	3.5	8.0
Cost of sales	3,068.4	2,742.3
Selling, General and Administrative Expense	358.4	324.7
Gain on sale of assets	0.0	(2.0)
Other (income) expense	(5.4)	(2.0)
Income (loss) before income taxes	461.6	354.3
Depreciation and amortization	16.5	14.7
Net cash (used in) provided by operating activities	(178.4)	(396.8)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	373.3		382.8
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	1,051.4		1,011.8
Land held for development	15.4		17.1
Land held for sale	0.0		0.0
Total inventories	1,066.8		1,028.9
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	11.8		17.4
Property and equipment, net	0.9		2.4
Other Assets	27.1		24.2
Goodwill	0.0		0.0
Total assets	1,479.9		1,455.7
Liabilities [Abstract]
Accounts payable	19.8		16.8
Accrued expenses and other liabilities	172.4		169.5
Notes payable	462.1		460.5
Total liabilities	654.3		646.8
Revenues
Total revenues	247.2	38.5
Inventory and land option charges	0.3	0.0
Cost of sales	216.6	30.7
Selling, General and Administrative Expense	10.5	5.7
Gain on sale of assets	0.1	(0.9)
Other (income) expense	(2.2)	(1.9)
Income (loss) before income taxes	22.2	4.9
Depreciation and amortization	0.1	0.1
Net cash (used in) provided by operating activities	(11.7)	(164.1)
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	19.6		25.1
Restricted cash	0.1		0.1
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	267.6		221.2
Other Assets	46.1		71.5
Goodwill	0.0		0.0
Total assets	333.4		317.9
Liabilities [Abstract]
Accounts payable	13.4		11.6
Accrued expenses and other liabilities	9.7		9.3
Notes payable	0.0		0.0
Total liabilities	23.1		20.9
Revenues
Total revenues	8.8	6.9
Inventory and land option charges	0.0	0.0
Cost of sales	0.0	0.0
Selling, General and Administrative Expense	8.8	6.7
Gain on sale of assets	(31.2)	0.0
Other (income) expense	1.6	(0.8)
Income (loss) before income taxes	29.6	1.0
Depreciation and amortization	1.9	1.6
Net cash (used in) provided by operating activities	3.9	(1.6)
East [Member] | Home Building [Member]
Inventories:
Total inventories	1,305.4		1,288.8
Revenues
Total revenues	520.5	447.5
Inventory and land option charges	0.1	1.4
Income (loss) before income taxes	59.9	38.0
Midwest [Member] | Home Building [Member]
Inventories:
Total inventories	919.0		836.8
Revenues
Total revenues	282.6	249.0
Inventory and land option charges	0.1	0.3
Income (loss) before income taxes	18.6	10.6
Southeast [Member] | Home Building [Member]
Inventories:
Total inventories	2,915.4		2,768.0
Revenues
Total revenues	1,150.6	1,013.9
Inventory and land option charges	1.4	1.2
Income (loss) before income taxes	146.3	112.3
South Central [Member] | Home Building [Member]
Inventories:
Total inventories	2,703.8		2,533.2
Revenues
Total revenues	958.7	872.5
Inventory and land option charges	1.5	0.5
Income (loss) before income taxes	132.6	106.0
Southwest [Member] | Home Building [Member]
Inventories:
Total inventories	612.1		574.4
Revenues
Total revenues	211.0	143.6
Inventory and land option charges	0.1	0.1
Income (loss) before income taxes	34.5	17.6
West [Member] | Home Building [Member]
Inventories:
Total inventories	2,195.6		2,056.0
Revenues
Total revenues	759.6	690.8
Inventory and land option charges	0.3	4.5
Income (loss) before income taxes	69.7	69.8
Corporate, Non-Segment [Member] | Home Building [Member]
Inventories:
Total inventories	234.3		228.4
Multifamily [Member] | Other Segments [Member]
Inventories:
Total assets	209.8		$ 204.0
Revenues
Gain on sale of assets	(31.2)
Financial Service [Member]
Revenues
Home sales	102.9	85.3
Financial Service [Member] | Segment Reconciling Items [Member]
Revenues
Home sales	0.0	0.0
Financial Service [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0
Financial Service [Member] | Financial Services [Member]
Revenues
Home sales	102.9	85.3
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	54.5	23.1
Cost of Goods and Services Sold	35.5	15.0
Land [Member] | Segment Reconciling Items [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(0.7)	3.6
Land [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	(221.2)	(29.0)
Cost of Goods and Services Sold	(193.4)	(24.4)
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	19.7	6.7
Cost of Goods and Services Sold	13.3	5.1
Land [Member] | Forestar Group [Member]
Revenues
Home sales	247.2	38.5
Cost of Goods and Services Sold	216.3	30.7
Land [Member] | Other Segments [Member]
Revenues
Home sales	8.8	6.9
Cost of Goods and Services Sold	0.0	0.0
Home Building [Member]
Revenues
Home sales	3,863.3	3,410.6
Cost of Goods and Services Sold	3,044.9	2,728.1
Home Building [Member] | Segment Reconciling Items [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	0.0	0.0
Home Building [Member] | Intersegment Eliminations [Member]
Revenues
Home sales	0.0	0.0
Cost of Goods and Services Sold	(6.7)	(1.1)
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	3,863.3	3,410.6
Cost of Goods and Services Sold	3,051.6	2,729.2
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