Segment Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Assets
Cash and cash equivalents	$ 3,210.4	$ 3,018.5
Restricted cash	26.8	21.6
Inventories:
Construction in progress and finished homes	7,739.2	5,984.1
Residential land and lots — developed and under development	7,781.8	6,171.8
Land held for development	110.9	53.2
Land held for sale	25.4	28.3
Rental properties	821.8	0.0
Total inventories	16,479.1	12,237.4
Mortgage loans held for sale	2,027.3	1,529.0
Deferred income taxes, net	155.3	144.9
Property and equipment, net	392.9	683.7
Other Assets	1,560.6	1,113.7
Goodwill	163.5	163.5
Total assets	24,015.9	18,912.3
Liabilities
Accounts payable	1,177.0	900.5
Accrued expenses and other liabilities	2,210.3	1,607.0
Notes payable	5,412.4	4,283.3
Total liabilities	8,799.7	6,790.8
Revenues:
Revenues	27,774.2	20,311.1	$ 17,592.9
Cost of sales:
Inventory and land option charges	28.6	23.8	54.0
Cost of sales	19,899.2	15,373.2	13,720.9
Selling, general and administrative expense	2,556.2	2,047.8	1,832.5
Gain on sale of assets	(14.0)	(59.5)	(53.9)
Loss on extinguishment of debt	18.1	0.0	0.0
Other (income) expense	(41.6)	(33.4)	(31.9)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	5,356.3	2,983.0	2,125.3
Depreciation and amortization	73.9	80.4	72.0
Cash provided by (used in) operating activities	534.4	1,421.6	892.1
Intersegment Eliminations [Member]
Assets
Cash and cash equivalents	10.9	4.9
Restricted cash	0.0	0.0
Inventories:
Construction in progress and finished homes	(108.8)	(53.4)
Residential land and lots — developed and under development	(46.9)	(33.9)
Land held for development	0.0	0.0
Land held for sale	0.0	0.0
Rental properties	(19.1)	(316.0)
Total inventories	(174.8)	(403.3)
Mortgage loans held for sale	0.0	0.0
Deferred income taxes, net	(3.9)	2.6
Property and equipment, net	82.6	392.0
Other Assets	(62.0)	(47.7)
Goodwill	29.2	29.2
Total assets	(118.0)	(22.3)
Liabilities
Accounts payable	0.0	0.0
Accrued expenses and other liabilities	(168.5)	(121.5)
Notes payable	(0.7)	(4.8)
Total liabilities	(169.2)	(126.3)
Revenues:
Revenues	(1,220.6)	(975.6)	(427.4)
Cost of sales:
Inventory and land option charges	0.0	0.0	0.0
Cost of sales	(1,188.4)	(896.5)	(359.0)
Selling, general and administrative expense	9.3	8.9	8.0
Gain on sale of assets	(11.5)	(59.4)	(48.9)
Loss on extinguishment of debt	0.0
Other (income) expense	24.2	17.0	11.2
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(54.2)	(45.6)	(38.7)
Depreciation and amortization	10.0	6.5	2.5
Cash provided by (used in) operating activities	203.5	66.3	(36.3)
Home Building
Assets
Cash and cash equivalents	2,950.1	2,551.0
Restricted cash	8.4	9.4
Inventories:
Construction in progress and finished homes	7,848.0	6,037.5
Residential land and lots — developed and under development	6,004.0	4,901.4
Land held for development	30.4	47.8
Land held for sale	25.4	28.3
Rental properties	0.0	0.0
Total inventories	13,907.8	11,015.0
Mortgage loans held for sale	0.0	0.0
Deferred income taxes, net	159.2	142.3
Property and equipment, net	303.3	286.7
Other Assets	1,468.7	996.3
Goodwill	134.3	134.3
Total assets	18,931.8	15,135.0
Liabilities
Accounts payable	1,073.7	858.1
Accrued expenses and other liabilities	1,941.3	1,437.5
Notes payable	3,214.0	2,514.4
Total liabilities	6,229.0	4,810.0
Revenues:
Revenues	26,577.6	19,641.5	17,016.9
Cost of sales:
Inventory and land option charges	24.9	22.9	53.2
Cost of sales	19,829.5	15,387.0	13,635.4
Selling, general and administrative expense	1,945.6	1,600.7	1,482.3
Gain on sale of assets	0.0	0.0	(2.0)
Loss on extinguishment of debt	0.0
Other (income) expense	(10.3)	(11.7)	(9.5)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	4,812.8	2,665.5	1,910.7
Depreciation and amortization	56.7	67.0	63.7
Cash provided by (used in) operating activities	1,239.8	1,888.4	1,452.5
Financial Services [Member]
Assets
Cash and cash equivalents	79.0	55.6
Restricted cash	18.0	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
Rental properties	0.0	0.0
Total inventories	0.0	0.0
Mortgage loans held for sale	2,027.3	1,529.0
Deferred income taxes, net	0.0	0.0
Property and equipment, net	3.5	3.9
Other Assets	107.6	125.8
Goodwill	0.0	0.0
Total assets	2,235.4	1,726.2
Liabilities
Accounts payable	0.0	0.0
Accrued expenses and other liabilities	88.6	86.8
Notes payable	1,494.6	1,132.6
Total liabilities	1,583.2	1,219.4
Revenues:
Revenues	823.6	584.9	441.7
Cost of sales:
Inventory and land option charges	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0
Selling, general and administrative expense	488.3	364.7	293.0
Gain on sale of assets	0.0	0.0	0.0
Loss on extinguishment of debt	0.0
Other (income) expense	(29.3)	(25.0)	(17.6)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	364.6	245.2	166.3
Depreciation and amortization	1.7	1.6	1.5
Cash provided by (used in) operating activities	(195.8)	(292.8)	(150.2)
Forestar Group [Member]
Assets
Cash and cash equivalents	153.6	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	1,824.7	1,304.3
Land held for development	80.5	5.4
Land held for sale	0.0	0.0
Rental properties	0.0	0.0
Total inventories	1,905.2	1,309.7
Mortgage loans held for sale	0.0	0.0
Deferred income taxes, net	0.0	0.0
Property and equipment, net	2.9	1.1
Other Assets	40.0	34.8
Goodwill	0.0	0.0
Total assets	2,101.7	1,739.9
Liabilities
Accounts payable	47.4	29.2
Accrued expenses and other liabilities	333.9	197.8
Notes payable	704.5	641.1
Total liabilities	1,085.8	868.1
Revenues:
Revenues	1,325.8	931.8	428.3
Cost of sales:
Inventory and land option charges	3.0	0.9	0.8
Cost of sales	1,096.6	813.7	362.7
Selling, general and administrative expense	68.4	45.7	28.9
Gain on sale of assets	(2.5)	(0.1)	(3.0)
Loss on extinguishment of debt	18.1
Other (income) expense	(1.4)	(5.6)	(6.0)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	146.6	78.1	45.7
Depreciation and amortization	0.4	0.3	0.2
Cash provided by (used in) operating activities	(303.1)	(168.5)	(391.3)
Rental
Assets
Cash and cash equivalents	16.8	12.7
Restricted cash	0.4	0.3
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
Rental properties	840.9	316.0
Total inventories	840.9	316.0
Mortgage loans held for sale	0.0	0.0
Deferred income taxes, net	0.0	0.0
Property and equipment, net	0.6	0.0
Other Assets	6.3	4.5
Goodwill	0.0	0.0
Total assets	865.0	333.5
Liabilities
Accounts payable	55.9	13.2
Accrued expenses and other liabilities	15.0	6.4
Notes payable	0.0	0.0
Total liabilities	70.9	19.6
Revenues:
Revenues	267.8	128.5	133.4
Cost of sales:
Inventory and land option charges	0.7	0.0	0.0
Cost of sales	161.5	69.0	81.8
Selling, general and administrative expense	44.6	27.8	20.3
Gain on sale of assets	0.0	0.0	0.0
Loss on extinguishment of debt	0.0
Other (income) expense	(24.8)	(8.1)	(10.0)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	86.5	39.8	41.3
Depreciation and amortization	5.1	5.0	4.1
Cash provided by (used in) operating activities	(410.0)	(71.8)	17.4
Northwest | Home Building
Inventories:
Total inventories	1,307.5	984.6
Goodwill	2.2	2.2
Revenues:
Revenues	2,516.6	1,953.4	1,721.5
Cost of sales:
Inventory and land option charges	0.6	1.3	6.3
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	510.8	264.5	222.9
Southwest [Member] | Home Building
Inventories:
Total inventories	2,445.6	2,063.3
Goodwill	0.0	0.0
Revenues:
Revenues	4,071.0	3,230.3	3,050.8
Cost of sales:
Inventory and land option charges	3.0	3.5	18.6
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	653.1	366.1	284.5
South Central [Member] | Home Building
Inventories:
Total inventories	3,479.3	2,577.6
Goodwill	15.9	15.9
Revenues:
Revenues	6,111.2	4,625.9	3,944.0
Cost of sales:
Inventory and land option charges	5.3	6.4	11.2
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,150.2	714.9	527.6
Southeast [Member] | Home Building
Inventories:
Total inventories	3,178.6	2,648.9
Goodwill	6.0	6.0
Revenues:
Revenues	7,079.6	4,871.5	4,023.5
Cost of sales:
Inventory and land option charges	10.2	5.9	8.3
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,371.9	709.5	477.8
East [Member] | Home Building
Inventories:
Total inventories	1,919.6	1,540.2
Goodwill	60.5	60.5
Revenues:
Revenues	4,459.0	3,410.1	3,081.2
Cost of sales:
Inventory and land option charges	2.4	3.0	3.9
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	795.1	484.3	361.9
North | Home Building
Inventories:
Total inventories	1,368.9	976.9
Goodwill	49.7	49.7
Revenues:
Revenues	2,340.2	1,550.3	1,195.9
Cost of sales:
Inventory and land option charges	3.4	2.8	4.9
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	331.7	126.2	36.0
Corporate, Non-Segment [Member] | Home Building
Inventories:
Total inventories	208.3	223.5
Home Building
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	26,502.6	19,560.8	16,925.0
Cost of sales:
Cost of Goods and Services Sold	19,608.3	15,247.2	13,498.8
Home Building | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	(140.1)	(58.6)	(8.3)
Home Building | Home Building
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	26,502.6	19,560.8	16,925.0
Cost of sales:
Cost of Goods and Services Sold	19,748.4	15,305.8	13,507.1
Home Building | Financial Services [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Home Building | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Home Building | Rental
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Land [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	212.0	165.4	226.2
Cost of sales:
Cost of Goods and Services Sold	119.3	102.2	168.1
Land [Member] | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	(1,188.8)	(847.1)	(294.0)
Cost of sales:
Cost of Goods and Services Sold	(1,030.5)	(768.9)	(268.9)
Land [Member] | Home Building
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	75.0	80.7	91.9
Cost of sales:
Cost of Goods and Services Sold	56.2	58.3	75.1
Land [Member] | Financial Services [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Land [Member] | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,325.8	931.8	428.3
Cost of sales:
Cost of Goods and Services Sold	1,093.6	812.8	361.9
Land [Member] | Rental
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Financial Services [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	823.6	584.9	441.7
Financial Services [Member] | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Financial Services [Member] | Home Building
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Financial Services [Member] | Financial Services [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	823.6	584.9	441.7
Financial Services [Member] | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Financial Services [Member] | Rental
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Rental
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	236.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	143.0	0.0	0.0
Rental | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	(31.8)	(128.5)	(133.4)
Cost of sales:
Cost of Goods and Services Sold	(17.8)	(69.0)	(81.8)
Rental | Home Building
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Rental | Financial Services [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Rental | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.0	0.0
Cost of sales:
Cost of Goods and Services Sold	0.0	0.0	0.0
Rental | Rental
Revenues:
Revenue from Contract with Customer, Including Assessed Tax	267.8	128.5	133.4
Cost of sales:
Cost of Goods and Services Sold	$ 160.8	$ 69.0	$ 81.8