Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Assets
Cash and cash equivalents	$ 1,494.3				$ 1,473.1				$ 1,494.3	$ 1,473.1
Restricted cash	19.7				32.9				19.7	32.9
Inventories:
Construction in progress and finished homes	5,245.0				5,086.3				5,245.0	5,086.3
Residential land and lots — developed and under development	5,939.4				5,172.4				5,939.4	5,172.4
Land held for development	77.8				96.1				77.8	96.1
Land held for sale	19.8				40.2				19.8	40.2
Total inventories	11,282.0				10,395.0				11,282.0	10,395.0
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	6.5				11.0				6.5	11.0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	1,072.0				796.4				1,072.0	796.4
Deferred income taxes, net	163.1				194.0				163.1	194.0
Property and equipment, net	462.2				401.1				462.2	401.1
Other Assets	943.3				701.9				943.3	701.9
Goodwill	163.5				109.2				163.5	109.2
Total assets	15,606.6				14,114.6				15,606.6	14,114.6
Liabilities
Accounts payable	634.0				624.7				634.0	624.7
Accrued expenses and other liabilities	1,278.1				1,127.5				1,278.1	1,127.5
Notes payable	3,399.4				3,203.5				3,399.4	3,203.5
Total liabilities	5,311.5				4,955.7				5,311.5	4,955.7
Revenues:
Revenues	5,038.9	$ 4,906.3	$ 4,128.7	$ 3,519.0	4,505.2	$ 4,435.3	$ 3,794.7	$ 3,332.7	17,592.9	16,068.0	$ 14,091.0
Cost of sales:
Inventory and land option charges									54.0	50.4	40.2
Cost of sales									13,720.9	12,398.1	11,042.8
Selling, general and administrative expense									1,832.5	1,676.8	1,471.6
Equity in earnings of unconsolidated entities									(0.5)	(2.8)	0.0
Gain on sale of assets									(53.9)	(18.8)	0.0
Interest expense										0.0
Other (income) expense									(31.4)	(45.3)	(25.5)
Income before income taxes	660.1	$ 626.7	$ 462.8	375.7	607.7	$ 616.2	$ 444.8	$ 391.2	2,125.3	2,060.0	1,602.1
Depreciation and amortization									72.0	62.4	54.7
Cash provided by (used in) operating activities									892.1	545.2	440.2
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	2.9				27.2				2.9	27.2
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	2.9				27.2				2.9	27.2
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	(0.8)				(0.7)				(0.8)	(0.7)
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0.0				0.0				0.0	0.0
Deferred income taxes, net	(5.8)				(10.5)				(5.8)	(10.5)
Property and equipment, net	0.0				0.0				0.0	0.0
Other Assets	11.9				12.6				11.9	12.6
Goodwill	29.2				29.2				29.2	29.2
Total assets	37.4				57.8				37.4	57.8
Liabilities
Accounts payable	0.0				0.0				0.0	0.0
Accrued expenses and other liabilities	(12.6)				(15.2)				(12.6)	(15.2)
Notes payable	2.4				8.2				2.4	8.2
Total liabilities	(10.2)				(7.0)				(10.2)	(7.0)
Revenues:
Revenues									0.0	(1.2)
Cost of sales:
Inventory and land option charges									0.0	0.6
Cost of sales									18.5	17.0
Selling, general and administrative expense									0.5	0.5
Equity in earnings of unconsolidated entities									0.0	7.1
Gain on sale of assets									3.0	24.7
Interest expense										0.0
Other (income) expense									0.0	1.0
Income before income taxes									(22.0)	(51.5)
Depreciation and amortization									0.5	0.5
Cash provided by (used in) operating activities									(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	(4.0)				1.9				(4.0)	1.9
Residential land and lots — developed and under development	(31.4)				(7.2)				(31.4)	(7.2)
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	(35.4)				(5.3)				(35.4)	(5.3)
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0.0				0.0				0.0	0.0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0.0				0.0				0.0	0.0
Deferred income taxes, net	5.1				1.1				5.1	1.1
Property and equipment, net	0.0				0.0				0.0	0.0
Other Assets	(88.5)				(48.6)				(88.5)	(48.6)
Goodwill	0.0				0.0				0.0	0.0
Total assets	(118.8)				(52.8)				(118.8)	(52.8)
Liabilities
Accounts payable	0.0				(3.3)				0.0	(3.3)
Accrued expenses and other liabilities	(93.6)				(46.1)				(93.6)	(46.1)
Notes payable	0.0				0.0				0.0	0.0
Total liabilities	(93.6)				(49.4)				(93.6)	(49.4)
Revenues:
Revenues									(326.6)	(39.1)
Cost of sales:
Inventory and land option charges									0.0	0.0
Cost of sales									(295.7)	(31.3)
Selling, general and administrative expense									0.0	0.0
Equity in earnings of unconsolidated entities									0.0	2.5
Gain on sale of assets									0.0	0.0
Interest expense										(5.8)
Other (income) expense									0.0	0.0
Income before income taxes									(30.9)	(4.5)
Depreciation and amortization									0.0	0.0
Cash provided by (used in) operating activities									(2.5)	(10.5)
Home Building [Member]
Assets
Cash and cash equivalents	1,043.0				1,111.8				1,043.0	1,111.8
Restricted cash	8.0				8.6				8.0	8.6
Inventories:
Construction in progress and finished homes	5,249.0				5,084.4				5,249.0	5,084.4
Residential land and lots — developed and under development	4,956.1				4,689.3				4,956.1	4,689.3
Land held for development	60.7				61.2				60.7	61.2
Land held for sale	19.8				40.2				19.8	40.2
Total inventories	10,285.6				9,875.1				10,285.6	9,875.1
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0.0				0.0				0.0	0.0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0.0				0.0				0.0	0.0
Deferred income taxes, net	146.4				176.5				146.4	176.5
Property and equipment, net	235.4				207.1				235.4	207.1
Other Assets	863.2				673.7				863.2	673.7
Goodwill	134.3				80.0				134.3	80.0
Total assets	12,715.9				12,132.8				12,715.9	12,132.8
Liabilities
Accounts payable	598.6				612.4				598.6	612.4
Accrued expenses and other liabilities	1,152.5				1,041.3				1,152.5	1,041.3
Notes payable	2,047.6				2,445.9				2,047.6	2,445.9
Total liabilities	3,798.7				4,099.6				3,798.7	4,099.6
Revenues:
Revenues									17,016.9	15,623.8	13,741.5
Cost of sales:
Inventory and land option charges									53.2	48.8	40.2
Cost of sales									13,635.4	12,343.4	11,042.8
Selling, general and administrative expense									1,482.3	1,346.2	1,220.4
Equity in earnings of unconsolidated entities									0.0	0.0
Gain on sale of assets									(2.0)	(15.8)
Interest expense										0.0
Other (income) expense									(9.5)	(7.2)	(11.0)
Income before income taxes									1,910.7	1,957.2	1,489.3
Depreciation and amortization									63.7	53.4	49.5
Cash provided by (used in) operating activities									1,438.0	1,001.7	303.7
Forestar Group [Member]
Assets
Cash and cash equivalents	382.8				318.8				382.8	318.8
Restricted cash	0.0				16.2				0.0	16.2
Inventories:
Construction in progress and finished homes	0.0				0.0				0.0	0.0
Residential land and lots — developed and under development	1,011.8				463.1				1,011.8	463.1
Land held for development	17.1				34.9				17.1	34.9
Land held for sale	0.0				0.0				0.0	0.0
Total inventories	1,028.9				498.0				1,028.9	498.0
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	7.3				11.7				7.3	11.7
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0.0				0.0				0.0	0.0
Deferred income taxes, net	17.4				26.9				17.4	26.9
Property and equipment, net	2.4				1.8				2.4	1.8
Other Assets	16.9				19.7				16.9	19.7
Goodwill	0.0				0.0				0.0	0.0
Total assets	1,455.7				893.1				1,455.7	893.1
Liabilities
Accounts payable	16.8				11.2				16.8	11.2
Accrued expenses and other liabilities	169.5				95.7				169.5	95.7
Notes payable	460.5				111.7				460.5	111.7
Total liabilities	646.8				218.6				646.8	218.6
Revenues:
Revenues									428.3	109.2
Cost of sales:
Inventory and land option charges									0.8	1.0
Cost of sales									362.7	69.0
Selling, general and administrative expense									28.9	32.8
Equity in earnings of unconsolidated entities									(0.5)	(12.4)
Gain on sale of assets									(3.0)	(27.7)
Interest expense										5.8
Other (income) expense									(5.5)	(7.0)
Income before income taxes									45.7	48.7
Depreciation and amortization									0.2	0.3
Cash provided by (used in) operating activities									(391.3)	(320.3)
Financial Services [Member]
Assets
Cash and cash equivalents	43.4				33.7				43.4	33.7
Restricted cash	11.6				8.1				11.6	8.1
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
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0.0				0.0				0.0	0.0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group					796.4					796.4
Deferred income taxes, net	0.0				0.0				0.0	0.0
Property and equipment, net	3.2				3.0				3.2	3.0
Other Assets	68.3				43.6				68.3	43.6
Goodwill	0.0				0.0				0.0	0.0
Total assets	1,198.5				884.8				1,198.5	884.8
Liabilities
Accounts payable	7.0				0.2				7.0	0.2
Accrued expenses and other liabilities	53.0				41.9				53.0	41.9
Notes payable	888.9				637.7				888.9	637.7
Total liabilities	948.9				679.8				948.9	679.8
Revenues:
Revenues									441.7	375.3	349.5
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									293.0	272.6	239.3
Equity in earnings of unconsolidated entities									0.0	0.0
Gain on sale of assets									0.0	0.0
Interest expense										0.0
Other (income) expense									(17.6)	(15.1)	(14.3)
Income before income taxes									166.3	117.8	124.5
Depreciation and amortization									1.5	1.4	1.5
Cash provided by (used in) operating activities									(150.2)	(116.6)	139.1
Other Segments [Member]
Assets
Cash and cash equivalents	25.1				8.8				25.1	8.8
Restricted cash	0.1				0.0				0.1	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	0.0				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
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0.0				0.0				0.0	0.0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0.0				0.0				0.0	0.0
Deferred income taxes, net	0.0				0.0				0.0	0.0
Property and equipment, net	221.2				189.2				221.2	189.2
Other Assets	71.5				0.9				71.5	0.9
Goodwill	0.0				0.0				0.0	0.0
Total assets	317.9				198.9				317.9	198.9
Liabilities
Accounts payable	11.6				4.2				11.6	4.2
Accrued expenses and other liabilities	9.3				9.9				9.3	9.9
Notes payable	0.0				0.0				0.0	0.0
Total liabilities	20.9				14.1				20.9	14.1
Revenues:
Revenues									32.6	0.0	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									27.8	24.7	11.9
Equity in earnings of unconsolidated entities									0.0	0.0
Gain on sale of assets									(51.9)	0.0
Interest expense										0.0
Other (income) expense									1.2	(17.0)	(0.2)
Income before income taxes									55.5	(7.7)	(11.7)
Depreciation and amortization									6.1	6.8	3.7
Cash provided by (used in) operating activities									2.5	0.8	(2.6)
East [Member] | Home Building [Member]
Inventories:
Total inventories	1,288.8				1,192.0				1,288.8	1,192.0
Goodwill	26.4			4.6	21.8				26.4	21.8
Revenues:
Revenues									2,290.2	1,893.4	1,640.1
Cost of sales:
Inventory and land option charges									2.7	2.3	13.6
Income before income taxes									238.8	217.3	153.9
Midwest [Member] | Home Building [Member]
Inventories:
Total inventories	836.8				583.1				836.8	583.1
Goodwill	49.7			$ 49.7	0.0				49.7	0.0
Revenues:
Revenues									1,123.1	858.9	736.5
Cost of sales:
Inventory and land option charges									3.5	5.1	1.8
Income before income taxes									57.7	77.5	49.1
Southeast [Member] | Home Building [Member]
Inventories:
Total inventories	2,768.0				2,668.7				2,768.0	2,668.7
Goodwill	40.1				40.1				40.1	40.1
Revenues:
Revenues									4,977.8	4,578.6	4,087.6
Cost of sales:
Inventory and land option charges									10.7	28.8	8.7
Income before income taxes									584.7	536.0	450.3
South Central [Member] | Home Building [Member]
Inventories:
Total inventories	2,533.2				2,439.4				2,533.2	2,439.4
Goodwill	15.9				15.9				15.9	15.9
Revenues:
Revenues									4,202.4	3,769.9	3,383.1
Cost of sales:
Inventory and land option charges									11.6	4.6	4.1
Income before income taxes									551.1	506.1	439.1
Southwest [Member] | Home Building [Member]
Inventories:
Total inventories	574.4				499.7				574.4	499.7
Goodwill	0.0				0.0				0.0	0.0
Revenues:
Revenues									772.6	768.7	597.5
Cost of sales:
Inventory and land option charges									0.5	0.9	1.6
Income before income taxes									100.4	97.4	39.6
West [Member] | Home Building [Member]
Inventories:
Total inventories	2,056.0				2,268.5				2,056.0	2,268.5
Goodwill	2.2				2.2				2.2	2.2
Revenues:
Revenues									3,650.8	3,754.3	3,296.7
Cost of sales:
Inventory and land option charges									24.2	7.1	10.4
Income before income taxes									378.0	522.9	357.3
Corporate, Non-Segment [Member] | Home Building [Member]
Inventories:
Total inventories	$ 228.4				$ 223.7				228.4	223.7
Home Building [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									16,925.0	15,502.0	13,653.2
Cost of sales:
Cost of Goods and Services Sold									13,498.8	12,194.3	10,927.8
Home Building [Member] | Segment Reconciling Items [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0
Cost of sales:
Cost of Goods and Services Sold									0.0	0.0
Home Building [Member] | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0
Cost of sales:
Cost of Goods and Services Sold									(8.3)	(1.2)
Home Building [Member] | Home Building [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									16,925.0	15,502.0	13,653.2
Cost of sales:
Cost of Goods and Services Sold									13,507.1	12,195.5	10,927.8
Home Building [Member] | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0
Cost of sales:
Cost of Goods and Services Sold									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									226.2	190.7	88.3
Cost of sales:
Cost of Goods and Services Sold									168.1	153.4	74.8
Land [Member] | Segment Reconciling Items [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	(1.2)
Cost of sales:
Cost of Goods and Services Sold									18.5	16.4
Land [Member] | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									(326.6)	(39.1)
Cost of sales:
Cost of Goods and Services Sold									(287.4)	(30.1)
Land [Member] | Home Building [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									91.9	121.8	88.3
Cost of sales:
Cost of Goods and Services Sold									75.1	99.1	74.8
Land [Member] | Forestar Group [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									428.3	109.2
Cost of sales:
Cost of Goods and Services Sold									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									32.6	0.0	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									441.7	375.3	349.5
Financial Service [Member] | Segment Reconciling Items [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									0.0	0.0
Financial Service [Member] | Intersegment Eliminations [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									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
Financial Service [Member] | Financial Services [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									441.7	375.3	349.5
Financial Service [Member] | Other Segments [Member]
Revenues:
Revenue from Contract with Customer, Including Assessed Tax									$ 0.0	$ 0.0	$ 0.0