Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Assets
Cash and cash equivalents	$ 3,018.5				$ 1,494.3				$ 3,018.5	$ 1,494.3
Restricted cash	21.6				19.7				21.6	19.7
Inventories:
Construction in progress and finished homes	5,984.1				5,245.0				5,984.1	5,245.0
Residential land and lots — developed and under development	6,171.8				5,939.4				6,171.8	5,939.4
Land held for development	53.2				77.8				53.2	77.8
Land held for sale	28.3				19.8				28.3	19.8
Total inventories	12,237.4				11,282.0				12,237.4	11,282.0
Mortgage loans held for sale	1,529.0				1,072.0				1,529.0	1,072.0
Deferred income taxes, net	144.9				163.1				144.9	163.1
Property and equipment, net	683.7				499.2				683.7	499.2
Other Assets	1,113.7				912.8				1,113.7	912.8
Goodwill	163.5				163.5				163.5	163.5
Total assets	18,912.3				15,606.6				18,912.3	15,606.6
Liabilities
Accounts payable	900.5				634.0				900.5	634.0
Accrued expenses and other liabilities	1,607.0				1,278.1				1,607.0	1,278.1
Notes payable	(4,283.3)				(3,399.4)				(4,283.3)	(3,399.4)
Total liabilities	6,790.8				5,311.5				6,790.8	5,311.5
Revenues:
Revenues	6,400.4	$ 5,390.0	$ 4,500.0	$ 4,020.7	5,038.9	$ 4,906.3	$ 4,128.7	$ 3,519.0	20,311.1	17,592.9	$ 16,068.0
Cost of sales:
Inventory and land option charges									23.8	54.0	50.4
Cost of sales									15,373.2	13,720.9	12,398.1
Selling, general and administrative expense									2,047.8	1,832.5	1,676.8
Equity in earnings of unconsolidated entities									(0.7)	(0.5)	(2.8)
Gain on sale of assets									(59.5)	(53.9)	(18.8)
Interest Expense											0.0
Other (income) expense									(32.7)	(31.4)	(45.3)
Income before income taxes	1,056.1	$ 782.4	$ 621.3	$ 523.3	660.1	$ 626.7	$ 462.8	$ 375.7	2,983.0	2,125.3	2,060.0
Depreciation and amortization									80.4	72.0	62.4
Cash provided by (used in) operating activities									1,421.6	892.1	545.2
Segment Reconciling Items [Member]
Assets
Cash and cash equivalents	0.0				0.0				0.0	0.0
Restricted cash	0.0				0.0				0.0	0.0
Inventories:
Construction in progress and finished homes	0.0				0.0				0.0	0.0
Residential land and lots — developed and under development	0.4				2.9				0.4	2.9
Land held for development	0.0				0.0				0.0	0.0
Land held for sale	0.0				0.0				0.0	0.0
Total inventories	0.4				2.9				0.4	2.9
Mortgage loans held for sale	0.0				0.0				0.0	0.0
Deferred income taxes, net	(5.8)				(5.8)				(5.8)	(5.8)
Property and equipment, net	0.0				0.0				0.0	0.0
Other Assets	7.5				11.1				7.5	11.1
Goodwill	29.2				29.2				29.2	29.2
Total assets	31.3				37.4				31.3	37.4
Liabilities
Accounts payable	0.0				0.0				0.0	0.0
Accrued expenses and other liabilities	(15.7)				(12.6)				(15.7)	(12.6)
Notes payable	0.0				(2.4)				0.0	(2.4)
Total liabilities	(15.7)				(10.2)				(15.7)	(10.2)
Revenues:
Revenues									0.0	0.0	(1.2)
Cost of sales:
Inventory and land option charges									0.0	0.0	0.6
Cost of sales									0.1	18.5	17.0
Selling, general and administrative expense									0.5	0.5	0.5
Equity in earnings of unconsolidated entities									0.0	0.0	7.1
Gain on sale of assets									0.0	3.0	24.7
Interest Expense											0.0
Other (income) expense									0.0	0.0	1.0
Income before income taxes									(0.6)	(22.0)	(51.5)
Depreciation and amortization									0.5	0.5	0.5
Cash provided by (used in) operating activities									0.0	(4.4)	(9.9)
Intersegment Eliminations [Member]
Assets
Cash and cash equivalents	0.0				0.0				0.0	0.0
Restricted cash	0.0				0.0				0.0	0.0
Inventories:
Construction in progress and finished homes	(53.4)				(4.0)				(53.4)	(4.0)
Residential land and lots — developed and under development	(34.3)				(31.4)				(34.3)	(31.4)
Land held for development	0.0				0.0				0.0	0.0
Land held for sale	0.0				0.0				0.0	0.0
Total inventories	(87.7)				(35.4)				(87.7)	(35.4)
Mortgage loans held for sale	0.0				0.0				0.0	0.0
Deferred income taxes, net	8.4				5.1				8.4	5.1
Property and equipment, net	(3.0)				0.0				(3.0)	0.0
Other Assets	(103.6)				(88.5)				(103.6)	(88.5)
Goodwill	0.0				0.0				0.0	0.0
Total assets	(185.9)				(118.8)				(185.9)	(118.8)
Liabilities
Accounts payable	0.0				0.0				0.0	0.0
Accrued expenses and other liabilities	(112.4)				(93.6)				(112.4)	(93.6)
Notes payable	(4.8)				0.0				(4.8)	0.0
Total liabilities	(117.2)				(93.6)				(117.2)	(93.6)
Revenues:
Revenues									(887.4)	(326.6)	(39.1)
Cost of sales:
Inventory and land option charges									0.0	0.0	0.0
Cost of sales									(827.6)	(295.7)	(31.3)
Selling, general and administrative expense									0.0	0.0	0.0
Equity in earnings of unconsolidated entities									0.0	0.0	2.5
Gain on sale of assets									0.0	0.0	0.0
Interest Expense											(5.8)
Other (income) expense									0.0	0.0	0.0
Income before income taxes									(59.8)	(30.9)	(4.5)
Depreciation and amortization									0.0	0.0	0.0
Cash provided by (used in) operating activities									(16.8)	(2.5)	(10.5)
Home Building [Member]
Assets
Cash and cash equivalents	2,551.1				1,043.0				2,551.1	1,043.0
Restricted cash	9.5				8.0				9.5	8.0
Inventories:
Construction in progress and finished homes	6,037.5				5,249.0				6,037.5	5,249.0
Residential land and lots — developed and under development	4,901.4				4,956.1				4,901.4	4,956.1
Land held for development	47.8				60.7				47.8	60.7
Land held for sale	28.3				19.8				28.3	19.8
Total inventories	11,015.0				10,285.6				11,015.0	10,285.6
Mortgage loans held for sale	0.0				0.0				0.0	0.0
Deferred income taxes, net	142.3				146.4				142.3	146.4
Property and equipment, net	372.8				272.4				372.8	272.4
Other Assets	996.4				826.2				996.4	826.2
Goodwill	134.3				134.3				134.3	134.3
Total assets	15,221.4				12,715.9				15,221.4	12,715.9
Liabilities
Accounts payable	859.3				598.6				859.3	598.6
Accrued expenses and other liabilities	1,438.3				1,152.5				1,438.3	1,152.5
Notes payable	(2,514.4)				(2,047.6)				(2,514.4)	(2,047.6)
Total liabilities	4,812.0				3,798.7				4,812.0	3,798.7
Revenues:
Revenues									19,643.9	17,016.9	15,623.8
Cost of sales:
Inventory and land option charges									22.9	53.2	48.8
Cost of sales									15,387.0	13,635.4	12,343.4
Selling, general and administrative expense									1,603.6	1,482.3	1,346.2
Equity in earnings of unconsolidated entities									0.0	0.0	0.0
Gain on sale of assets									0.0	(2.0)	(15.8)
Interest Expense											0.0
Other (income) expense									(11.7)	(9.5)	(7.2)
Income before income taxes									2,665.0	1,910.7	1,957.2
Depreciation and amortization									68.2	63.7	53.4
Cash provided by (used in) operating activities									1,904.2	1,438.0	1,001.7
Forestar Group [Member]
Assets
Cash and cash equivalents	394.3				382.8				394.3	382.8
Restricted cash	0.0				0.0				0.0	0.0
Inventories:
Construction in progress and finished homes	0.0				0.0				0.0	0.0
Residential land and lots — developed and under development	1,304.3				1,011.8				1,304.3	1,011.8
Land held for development	5.4				17.1				5.4	17.1
Land held for sale	0.0				0.0				0.0	0.0
Total inventories	1,309.7				1,028.9				1,309.7	1,028.9
Mortgage loans held for sale	0.0				0.0				0.0	0.0
Deferred income taxes, net	0.0				17.4				0.0	17.4
Property and equipment, net	1.1				2.4				1.1	2.4
Other Assets	34.8				24.2				34.8	24.2
Goodwill	0.0				0.0				0.0	0.0
Total assets	1,739.9				1,455.7				1,739.9	1,455.7
Liabilities
Accounts payable	29.2				16.8				29.2	16.8
Accrued expenses and other liabilities	197.8				169.5				197.8	169.5
Notes payable	(641.1)				(460.5)				(641.1)	(460.5)
Total liabilities	868.1				646.8				868.1	646.8
Revenues:
Revenues									931.8	428.3	109.2
Cost of sales:
Inventory and land option charges									0.9	0.8	1.0
Cost of sales									813.7	362.7	69.0
Selling, general and administrative expense									45.7	28.9	32.8
Equity in earnings of unconsolidated entities									(0.7)	(0.5)	(12.4)
Gain on sale of assets									(0.1)	(3.0)	(27.7)
Interest Expense											5.8
Other (income) expense									(4.9)	(5.5)	(7.0)
Income before income taxes									78.1	45.7	48.7
Depreciation and amortization									0.3	0.2	0.3
Cash provided by (used in) operating activities									(168.5)	(391.3)	(320.3)
Financial Services [Member]
Assets
Cash and cash equivalents	55.6				43.4				55.6	43.4
Restricted cash	11.9				11.6				11.9	11.6
Inventories:
Construction in progress and finished homes	0.0				0.0				0.0	0.0
Residential land and lots — developed and under development	0.0				0.0				0.0	0.0
Land held for development	0.0				0.0				0.0	0.0
Land held for sale	0.0				0.0				0.0	0.0
Total inventories	0.0				0.0				0.0	0.0
Mortgage loans held for sale	1,529.0				1,072.0				1,529.0	1,072.0
Deferred income taxes, net	0.0				0.0				0.0	0.0
Property and equipment, net	3.9				3.2				3.9	3.2
Other Assets	125.8				68.3				125.8	68.3
Goodwill	0.0				0.0				0.0	0.0
Total assets	1,726.2				1,198.5				1,726.2	1,198.5
Liabilities
Accounts payable	0.0				7.0				0.0	7.0
Accrued expenses and other liabilities	86.8				53.0				86.8	53.0
Notes payable	(1,132.6)				(888.9)				(1,132.6)	(888.9)
Total liabilities	1,219.4				948.9				1,219.4	948.9
Revenues:
Revenues									584.9	441.7	375.3
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									364.7	293.0	272.6
Equity in earnings of unconsolidated entities									0.0	0.0	0.0
Gain on sale of assets									0.0	0.0	0.0
Interest Expense											0.0
Other (income) expense									(25.0)	(17.6)	(15.1)
Income before income taxes									245.2	166.3	117.8
Depreciation and amortization									1.6	1.5	1.4
Cash provided by (used in) operating activities									(292.8)	(150.2)	(116.6)
Other Segments [Member]
Assets
Cash and cash equivalents	17.5				25.1				17.5	25.1
Restricted cash	0.2				0.1				0.2	0.1
Inventories:
Construction in progress and finished homes	0.0				0.0				0.0	0.0
Residential land and lots — developed and under development	0.0				0.0				0.0	0.0
Land held for development	0.0				0.0				0.0	0.0
Land held for sale	0.0				0.0				0.0	0.0
Total inventories	0.0				0.0				0.0	0.0
Mortgage loans held for sale	0.0				0.0				0.0	0.0
Deferred income taxes, net	0.0				0.0				0.0	0.0
Property and equipment, net	308.9				221.2				308.9	221.2
Other Assets	52.8				71.5				52.8	71.5
Goodwill	0.0				0.0				0.0	0.0
Total assets	379.4				317.9				379.4	317.9
Liabilities
Accounts payable	12.0				11.6				12.0	11.6
Accrued expenses and other liabilities	12.2				9.3				12.2	9.3
Notes payable	0.0				0.0				0.0	0.0
Total liabilities	24.2				20.9				24.2	20.9
Revenues:
Revenues									37.9	32.6	0.0
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									33.3	27.8	24.7
Equity in earnings of unconsolidated entities									0.0	0.0	0.0
Gain on sale of assets									(59.4)	(51.9)	0.0
Interest Expense											0.0
Other (income) expense									8.9	1.2	(17.0)
Income before income taxes									55.1	55.5	(7.7)
Depreciation and amortization									9.8	6.1	6.8
Cash provided by (used in) operating activities									(4.5)	2.5	0.8
East [Member] | Home Building [Member]
Inventories:
Total inventories	1,328.3				1,288.8				1,328.3	1,288.8
Goodwill	26.4				26.4				26.4	26.4
Revenues:
Revenues									2,642.3	2,290.2	1,893.4
Cost of sales:
Inventory and land option charges									1.8	2.7	2.3
Income before income taxes									357.0	238.8	217.3
Midwest [Member] | Home Building [Member]
Inventories:
Total inventories	958.5				836.8				958.5	836.8
Goodwill	49.7				49.7				49.7	49.7
Revenues:
Revenues									1,429.0	1,123.1	858.9
Cost of sales:
Inventory and land option charges									2.2	3.5	5.1
Income before income taxes									128.8	57.7	77.5
Southeast [Member] | Home Building [Member]
Inventories:
Total inventories	2,919.9				2,768.0				2,919.9	2,768.0
Goodwill	40.1				40.1				40.1	40.1
Revenues:
Revenues									5,845.2	4,977.8	4,578.6
Cost of sales:
Inventory and land option charges									7.8	10.7	28.8
Income before income taxes									841.0	584.7	536.0
South Central [Member] | Home Building [Member]
Inventories:
Total inventories	2,879.9				2,533.2				2,879.9	2,533.2
Goodwill	15.9				15.9				15.9	15.9
Revenues:
Revenues									4,998.1	4,202.4	3,769.9
Cost of sales:
Inventory and land option charges									6.7	11.6	4.6
Income before income taxes									758.0	551.1	506.1
Southwest [Member] | Home Building [Member]
Inventories:
Total inventories	695.8				574.4				695.8	574.4
Goodwill	0.0				0.0				0.0	0.0
Revenues:
Revenues									881.6	772.6	768.7
Cost of sales:
Inventory and land option charges									0.1	0.5	0.9
Income before income taxes									136.9	100.4	97.4
West [Member] | Home Building [Member]
Inventories:
Total inventories	2,009.1				2,056.0				2,009.1	2,056.0
Goodwill	2.2				2.2				2.2	2.2
Revenues:
Revenues									3,847.7	3,650.8	3,754.3
Cost of sales:
Inventory and land option charges									4.3	24.2	7.1
Income before income taxes									443.3	378.0	522.9
Corporate, Non-Segment [Member] | Home Building [Member]
Inventories:
Total inventories	$ 223.5				$ 228.4				223.5	228.4
Home Building [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									19,560.8	16,925.0	15,502.0
Cost of sales:
Cost of Goods and Services Sold									15,247.2	13,498.8	12,194.3
Home Building [Member] | Segment Reconciling Items [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 [Member] | 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									(58.6)	(8.3)	(1.2)
Home Building [Member] | Home Building [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									19,560.8	16,925.0	15,502.0
Cost of sales:
Cost of Goods and Services Sold									15,305.8	13,507.1	12,195.5
Home Building [Member] | 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 [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
Home Building [Member] | Other Segments [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]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									165.4	226.2	190.7
Cost of sales:
Cost of Goods and Services Sold									102.2	168.1	153.4
Land [Member] | Segment Reconciling Items [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0	(1.2)
Cost of sales:
Cost of Goods and Services Sold									0.1	18.5	16.4
Land [Member] | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									(887.4)	(326.6)	(39.1)
Cost of sales:
Cost of Goods and Services Sold									(769.0)	(287.4)	(30.1)
Land [Member] | Home Building [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									83.1	91.9	121.8
Cost of sales:
Cost of Goods and Services Sold									58.3	75.1	99.1
Land [Member] | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									931.8	428.3	109.2
Cost of sales:
Cost of Goods and Services Sold									812.8	361.9	68.0
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] | Other Segments [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									37.9	32.6	0.0
Cost of sales:
Cost of Goods and Services Sold									0.0	0.0	0.0
Financial Service [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									584.9	441.7	375.3
Financial Service [Member] | Segment Reconciling Items [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0	0.0
Financial Service [Member] | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0	0.0
Financial Service [Member] | Home Building [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0	0.0
Financial Service [Member] | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0	0.0
Financial Service [Member] | Financial Services [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									584.9	441.7	375.3
Financial Service [Member] | Other Segments [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									$ 0.0	$ 0.0	$ 0.0