Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
Assets [Abstract]
Cash and cash equivalents	$ 1,178.2	$ 512.4	$ 1,178.2	$ 512.4	$ 1,007.8	$ 1,303.2
Restricted cash	58.7		58.7		16.5
Inventories:
Construction in progress and finished homes	5,194.8		5,194.8		4,606.0
Residential land and lots - developed and under development	4,948.2		4,948.2		4,519.7
Land held for development	117.0		117.0		101.0
Land held for sale	43.3		43.3		10.4
Total inventories	10,303.3		10,303.3		9,237.1
Investment in unconsolidated entities	32.2		32.2		0.0
Mortgage loans held for sale	679.9		679.9		587.3
Deferred income taxes, net	204.6		204.6		365.0
Property and equipment, net	385.2		385.2		325.0
Other Assets	642.1		642.1		565.9
Goodwill	109.2		109.2		80.0
Total assets	13,593.4		13,593.4		12,184.6
Liabilities [Abstract]
Accounts payable	655.3		655.3		580.4
Accrued expenses and other liabilities	1,074.6		1,074.6		985.0
Notes payable	3,093.6		3,093.6		2,871.6
Total liabilities	4,823.5		4,823.5		4,437.0
Revenues
Home sales	4,265.5	3,662.3	11,122.1	9,618.1
Land/lot sales and other	72.7	22.2	167.7	56.9
Financial services	97.1	91.9	273.1	256.9
Total revenues	4,435.3	3,776.4	11,562.9	9,931.9
Cost of sales
Home sales	3,332.8	2,936.9	8,761.7	7,713.8
Land/lot sales and other	55.5	18.8	134.5	45.2
Inventory and land option charges	8.9	5.4	42.8	19.9
Total cost of sales	3,397.2	2,961.1	8,939.0	7,778.9
Selling, General and Administrative Expense	434.9	374.5	1,219.9	1,055.5
Equity in earnings of unconsolidated entities	(0.4)	0.0	(3.1)	0.0
Gain on sale of assets	0.0	0.0	(14.5)	0.0
Interest Expense	0.0		0.0
Other (income) expense	(12.6)	(3.7)	(30.7)	(19.0)
Income (loss) before income taxes	616.2	444.5	1,452.3	1,116.5
Depreciation and amortization			46.6	40.4
Net cash (used in) provided by operating activities			306.5	(185.7)
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	31.8		31.8
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	31.8		31.8
Investment in unconsolidated entities	14.2		14.2
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	0.1		0.1
Property and equipment, net	0.0		0.0
Other Assets	18.8		18.8
Goodwill	29.2		29.2
Total assets	94.1		94.1
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	(20.8)		(20.8)
Notes payable	9.7		9.7
Total liabilities	(11.1)		(11.1)
Revenues
Home sales	0.0		0.0
Land/lot sales and other	(1.2)		(1.2)
Financial services	0.0		0.0
Total revenues	(1.2)		(1.2)
Cost of sales
Home sales	0.0		0.0
Land/lot sales and other	5.7		12.6
Inventory and land option charges	0.0		0.0
Total cost of sales	5.7		12.6
Selling, General and Administrative Expense	0.1		0.3
Equity in earnings of unconsolidated entities	0.6		7.0
Gain on sale of assets	1.3		2.9
Interest Expense	0.0		0.0
Other (income) expense	0.7		0.7
Income (loss) before income taxes	(9.6)		(24.7)
Depreciation and amortization			0.4
Net cash (used in) provided by operating activities			(8.1)
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	0.0		0.0
Residential land and lots - developed and under development	0.4		0.4
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	0.4		0.4
Investment in unconsolidated entities	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	0.0		0.0
Other Assets	(39.5)		(39.5)
Goodwill	0.0		0.0
Total assets	(39.1)		(39.1)
Liabilities [Abstract]
Accounts payable	(3.2)		(3.2)
Accrued expenses and other liabilities	(36.7)		(36.7)
Notes payable	0.0		0.0
Total liabilities	(39.9)		(39.9)
Revenues
Home sales	0.0		0.0
Land/lot sales and other	(8.8)		(17.3)
Financial services	0.0		0.0
Total revenues	(8.8)		(17.3)
Cost of sales
Home sales	0.0		0.0
Land/lot sales and other	(5.6)		(12.3)
Inventory and land option charges	0.0		0.0
Total cost of sales	(5.6)		(12.3)
Selling, General and Administrative Expense	0.0		0.0
Equity in earnings of unconsolidated entities	0.0		0.0
Gain on sale of assets	0.0		0.0
Interest Expense	(1.6)		(5.8)
Other (income) expense	0.0		0.0
Income (loss) before income taxes	(1.6)		0.8
Depreciation and amortization			0.0
Net cash (used in) provided by operating activities			0.0
Homebuilding [Member]
Assets [Abstract]
Cash and cash equivalents	748.0		748.0		973.0
Restricted cash	11.2		11.2		9.3
Inventories:
Construction in progress and finished homes	5,194.8		5,194.8		4,606.0
Residential land and lots - developed and under development	4,590.1		4,590.1		4,519.7
Land held for development	82.1		82.1		101.0
Land held for sale	43.3		43.3		10.4
Total inventories	9,910.3		9,910.3		9,237.1
Investment in unconsolidated entities	0.0		0.0
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	203.2		203.2		365.0
Property and equipment, net	205.5		205.5		194.4
Other Assets	594.2		594.2		518.7
Goodwill	80.0		80.0		80.0
Total assets	11,752.4		11,752.4		11,377.5
Liabilities [Abstract]
Accounts payable	641.8		641.8		575.6
Accrued expenses and other liabilities	1,007.6		1,007.6		933.1
Notes payable	2,447.1		2,447.1		2,451.6
Total liabilities	4,096.5		4,096.5		3,960.3
Revenues
Home sales	4,265.5	3,662.3	11,122.1	9,618.1
Land/lot sales and other	59.1	22.2	109.2	56.9
Financial services	0.0	0.0	0.0	0.0
Total revenues	4,324.6	3,684.5	11,231.3	9,675.0
Cost of sales
Home sales	3,332.8	2,936.9	8,761.7	7,713.8
Land/lot sales and other	45.4	18.8	88.7	45.2
Inventory and land option charges	8.9	5.4	42.8	19.9
Total cost of sales	3,387.1	2,961.1	8,893.2	7,778.9
Selling, General and Administrative Expense	349.1	309.5	976.6	872.4
Equity in earnings of unconsolidated entities	0.0		0.0
Gain on sale of assets	0.0		(13.4)
Interest Expense	0.0		0.0
Other (income) expense	(1.3)	(1.3)	(4.6)	(7.8)
Income (loss) before income taxes	589.7	415.2	1,379.5	1,031.5
Depreciation and amortization			39.8	36.8
Net cash (used in) provided by operating activities			565.2	(259.6)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	367.7		367.7
Restricted cash	40.0		40.0
Inventories:
Construction in progress and finished homes	0.0		0.0
Residential land and lots - developed and under development	325.9		325.9
Land held for development	34.9		34.9
Land held for sale	0.0		0.0
Total inventories	360.8		360.8
Investment in unconsolidated entities	18.0		18.0
Mortgage loans held for sale	0.0		0.0
Deferred income taxes, net	1.3		1.3
Property and equipment, net	1.8		1.8
Other Assets	21.2		21.2
Goodwill	0.0		0.0
Total assets	810.8		810.8
Liabilities [Abstract]
Accounts payable	7.4		7.4
Accrued expenses and other liabilities	72.8		72.8
Notes payable	110.5		110.5
Total liabilities	190.7		190.7
Revenues
Home sales	0.0		0.0
Land/lot sales and other	23.6		77.0
Financial services	0.0		0.0
Total revenues	23.6		77.0
Cost of sales
Home sales	0.0		0.0
Land/lot sales and other	10.0		45.5
Inventory and land option charges	0.0		0.0
Total cost of sales	10.0		45.5
Selling, General and Administrative Expense	6.5		25.6
Equity in earnings of unconsolidated entities	(1.0)		(10.1)
Gain on sale of assets	(1.3)		(4.0)
Interest Expense	1.6		5.8
Other (income) expense	(2.7)		(4.9)
Income (loss) before income taxes	10.5		19.1
Depreciation and amortization			0.2
Net cash (used in) provided by operating activities			(219.2)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	43.1		43.1		24.1
Restricted cash	7.5		7.5		7.2
Inventories:
Construction in progress and finished homes	0.0		0.0		0.0
Residential land and lots - developed and under development	0.0		0.0		0.0
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Total inventories	0.0		0.0		0.0
Investment in unconsolidated entities	0.0		0.0
Mortgage loans held for sale	679.9		679.9		587.3
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	3.0		3.0		3.0
Other Assets	44.0		44.0		42.2
Goodwill	0.0		0.0		0.0
Total assets	777.5		777.5		663.8
Liabilities [Abstract]
Accounts payable	3.9		3.9		1.5
Accrued expenses and other liabilities	38.9		38.9		35.6
Notes payable	526.3		526.3		420.0
Total liabilities	569.1		569.1		457.1
Revenues
Home sales	0.0	0.0	0.0	0.0
Land/lot sales and other	0.0	0.0	0.0	0.0
Financial services	97.1	91.9	273.1	256.9
Total revenues	97.1	91.9	273.1	256.9
Cost of sales
Home sales	0.0	0.0	0.0	0.0
Land/lot sales and other	0.0	0.0	0.0	0.0
Inventory and land option charges	0.0	0.0	0.0	0.0
Total cost of sales	0.0	0.0	0.0	0.0
Selling, General and Administrative Expense	71.1	62.1	199.6	175.0
Equity in earnings of unconsolidated entities	0.0		0.0
Gain on sale of assets	0.0		0.0
Interest Expense	0.0		0.0
Other (income) expense	(4.3)	(4.1)	(10.5)	(10.8)
Income (loss) before income taxes	30.3	33.9	84.0	92.7
Depreciation and amortization			1.0	1.0
Net cash (used in) provided by operating activities			(27.0)	76.9
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	19.4		19.4		10.7
Restricted cash	0.0		0.0		0.0
Inventories:
Construction in progress and finished homes	0.0		0.0		0.0
Residential land and lots - developed and under development	0.0		0.0		0.0
Land held for development	0.0		0.0		0.0
Land held for sale	0.0		0.0		0.0
Total inventories	0.0		0.0		0.0
Investment in unconsolidated entities	0.0		0.0
Mortgage loans held for sale	0.0		0.0		0.0
Deferred income taxes, net	0.0		0.0		0.0
Property and equipment, net	174.9		174.9		127.6
Other Assets	3.4		3.4		5.0
Goodwill	0.0		0.0		0.0
Total assets	197.7		197.7		143.3
Liabilities [Abstract]
Accounts payable	5.4		5.4		3.3
Accrued expenses and other liabilities	12.8		12.8		16.3
Notes payable	0.0		0.0		0.0
Total liabilities	18.2		18.2		19.6
Revenues
Home sales	0.0	0.0	0.0	0.0
Land/lot sales and other	0.0	0.0	0.0	0.0
Financial services	0.0	0.0	0.0	0.0
Total revenues	0.0	0.0	0.0	0.0
Cost of sales
Home sales	0.0	0.0	0.0	0.0
Land/lot sales and other	0.0	0.0	0.0	0.0
Inventory and land option charges	0.0	0.0	0.0	0.0
Total cost of sales	0.0	0.0	0.0	0.0
Selling, General and Administrative Expense	8.1	2.9	17.8	8.1
Equity in earnings of unconsolidated entities	0.0		0.0
Gain on sale of assets	0.0		0.0
Interest Expense	0.0		0.0
Other (income) expense	(5.0)	1.7	(11.4)	(0.4)
Income (loss) before income taxes	(3.1)	(4.6)	(6.4)	(7.7)
Depreciation and amortization			5.2	2.6
Net cash (used in) provided by operating activities			(4.4)	(3.0)
East [Member] | Homebuilding [Member]
Inventories:
Total inventories	1,211.3		1,211.3		1,068.9
Revenues
Total revenues	529.1	482.2	1,357.9	1,160.7
Cost of sales
Inventory and land option charges	0.3	0.5	0.9	7.0
Income (loss) before income taxes	67.1	54.2	158.7	106.4
Midwest [Member] | Homebuilding [Member]
Inventories:
Total inventories	540.3		540.3		492.6
Revenues
Total revenues	256.7	202.2	621.7	522.0
Cost of sales
Inventory and land option charges	4.3	1.3	4.7	1.6
Income (loss) before income taxes	23.0	17.9	55.0	28.8
Southeast [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,652.2		2,652.2		2,392.3
Revenues
Total revenues	1,263.3	1,136.3	3,293.9	2,988.7
Cost of sales
Inventory and land option charges	1.1	2.3	27.3	3.9
Income (loss) before income taxes	156.1	128.6	374.9	341.3
South Central [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,407.9		2,407.9		2,199.4
Revenues
Total revenues	1,005.4	924.8	2,733.2	2,497.1
Cost of sales
Inventory and land option charges	1.5	0.4	3.5	2.3
Income (loss) before income taxes	143.3	119.1	365.2	321.4
Southwest [Member] | Homebuilding [Member]
Inventories:
Total inventories	527.5		527.5		506.1
Revenues
Total revenues	220.1	152.6	548.6	387.8
Cost of sales
Inventory and land option charges	0.0	0.0	0.8	0.1
Income (loss) before income taxes	32.3	14.8	69.1	26.0
West [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,346.1		2,346.1		2,352.5
Revenues
Total revenues	1,050.0	786.4	2,676.0	2,118.7
Cost of sales
Inventory and land option charges	1.7	0.9	5.6	5.0
Income (loss) before income taxes	167.9	$ 80.6	356.6	$ 207.6
Corporate, Non-Segment [Member] | Homebuilding [Member]
Inventories:
Total inventories	$ 225.0		$ 225.0		$ 225.3