Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017	Sep. 30, 2016
Assets [Abstract]
Cash and cash equivalents	$ 1,010.8	$ 993.2	$ 1,010.8	$ 993.2	$ 1,007.8	$ 1,303.2
Restricted cash	55.4		55.4		16.5
Inventories:
Construction in progress and finished homes	5,119.6		5,119.6		4,606.0
Residential land and lots - developed and under development	4,900.0		4,900.0		4,519.7
Land held for development	86.6		86.6		101.0
Land held for sale	38.2		38.2		10.4
Total inventories	10,144.4		10,144.4		9,237.1
Investment in unconsolidated entities	32.1		32.1		0.0
Mortgage loans held for sale	658.2		658.2		587.3
Deferred income taxes	219.9		219.9		365.0
Property and equipment, net	380.4		380.4		325.0
Other Assets	607.7		607.7		565.9
Goodwill	109.2		109.2		80.0
Total assets	13,218.1		13,218.1		12,184.6
Liabilities [Abstract]
Accounts payable	582.7		582.7		580.4
Accrued expenses and other liabilities	1,029.6		1,029.6		985.0
Notes payable	3,233.9		3,233.9		2,871.6
Total liabilities	4,846.2		4,846.2		4,437.0
Revenues
Home sales	3,672.1	3,158.1	6,856.6	5,955.8
Land/lot sales and other	27.7	6.3	95.0	34.7
Financial services	94.9	86.9	176.0	165.0
Total revenues	3,794.7	3,251.3	7,127.6	6,155.5
Cost of sales
Home sales	2,907.5	2,532.1	5,429.0	4,776.9
Land/lot sales and other	24.0	5.6	79.0	26.4
Inventory and land option charges	30.1	12.2	33.8	14.5
Total cost of sales	2,961.6	2,549.9	5,541.8	4,817.8
Selling, General and Administrative Expense	400.9	355.2	785.1	681.0
Equity in earnings of unconsolidated entities	(0.4)	0.0	(2.7)	0.0
Gain on sale of assets	(1.1)	0.0	(14.5)	0.0
Interest Expense	0.0		0.0
Other (income) expense	(11.1)	(7.7)	(18.2)	(15.4)
Income (loss) before income taxes	444.8	353.9	836.1	672.1
Depreciation and amortization			33.1	27.3
Net cash (used in) provided by operating activities			(98.8)	(135.7)
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	40.7		40.7
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	40.7		40.7
Investment in unconsolidated entities	14.8		14.8
Mortgage loans held for sale	0.0		0.0
Deferred income taxes	0.3		0.3
Property and equipment, net	0.0		0.0
Other Assets	18.7		18.7
Goodwill	29.2		29.2
Total assets	103.7		103.7
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	(25.3)		(25.3)
Notes payable	11.2		11.2
Total liabilities	(14.1)		(14.1)
Revenues
Home sales	0.0		0.0
Land/lot sales and other	0.0		0.0
Financial services	0.0		0.0
Total revenues	0.0		0.0
Cost of sales
Home sales	0.0		0.0
Land/lot sales and other	2.5		6.9
Inventory and land option charges	0.0		0.0
Total cost of sales	2.5		6.9
Selling, General and Administrative Expense	0.1		0.3
Equity in earnings of unconsolidated entities	1.1		6.4
Gain on sale of assets	1.6		1.6
Interest Expense	0.0		0.0
Other (income) expense	0.0		0.0
Income (loss) before income taxes	(5.3)		(15.2)
Depreciation and amortization			0.3
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	2.4		2.4
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	2.4		2.4
Investment in unconsolidated entities	0.0		0.0
Mortgage loans held for sale	0.0		0.0
Deferred income taxes	0.0		0.0
Property and equipment, net	0.0		0.0
Other Assets	(23.0)		(23.0)
Goodwill	0.0		0.0
Total assets	(20.6)		(20.6)
Liabilities [Abstract]
Accounts payable	0.0		0.0
Accrued expenses and other liabilities	(23.0)		(23.0)
Notes payable	0.0		0.0
Total liabilities	(23.0)		(23.0)
Revenues
Home sales	0.0		0.0
Land/lot sales and other	(8.5)		(8.5)
Financial services	0.0		0.0
Total revenues	(8.5)		(8.5)
Cost of sales
Home sales	0.0		0.0
Land/lot sales and other	(6.7)		(6.7)
Inventory and land option charges	0.0		0.0
Total cost of sales	(6.7)		(6.7)
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	(2.1)		(4.2)
Other (income) expense	0.0		0.0
Income (loss) before income taxes	0.3		2.4
Depreciation and amortization			0.0
Net cash (used in) provided by operating activities			0.0
Homebuilding [Member]
Assets [Abstract]
Cash and cash equivalents	528.9		528.9		973.0
Restricted cash	9.4		9.4		9.3
Inventories:
Construction in progress and finished homes	5,119.6		5,119.6		4,606.0
Residential land and lots - developed and under development	4,595.2		4,595.2		4,519.7
Land held for development	86.6		86.6		101.0
Land held for sale	38.2		38.2		10.4
Total inventories	9,839.6		9,839.6		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	218.2		218.2		365.0
Property and equipment, net	204.8		204.8		194.4
Other Assets	543.6		543.6		518.7
Goodwill	80.0		80.0		80.0
Total assets	11,424.5		11,424.5		11,377.5
Liabilities [Abstract]
Accounts payable	568.9		568.9		575.6
Accrued expenses and other liabilities	967.9		967.9		933.1
Notes payable	2,623.1		2,623.1		2,451.6
Total liabilities	4,159.9		4,159.9		3,960.3
Revenues
Home sales	3,672.1	3,158.1	6,856.6	5,955.8
Land/lot sales and other	13.6	6.3	50.0	34.7
Financial services	0.0	0.0	0.0	0.0
Total revenues	3,685.7	3,164.4	6,906.6	5,990.5
Cost of sales
Home sales	2,907.5	2,532.1	5,429.0	4,776.9
Land/lot sales and other	12.0	5.6	43.3	26.4
Inventory and land option charges	30.1	12.2	33.8	14.5
Total cost of sales	2,949.6	2,549.9	5,506.1	4,817.8
Selling, General and Administrative Expense	322.7	294.5	627.5	562.9
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	(2.6)	(2.4)	(3.4)	(6.5)
Income (loss) before income taxes	416.0	322.4	789.8	616.3
Depreciation and amortization			26.3	25.0
Net cash (used in) provided by operating activities			90.7	(240.4)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	436.4		436.4
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	261.7		261.7
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	261.7		261.7
Investment in unconsolidated entities	17.3		17.3
Mortgage loans held for sale	0.0		0.0
Deferred income taxes	1.4		1.4
Property and equipment, net	1.8		1.8
Other Assets	22.5		22.5
Goodwill	0.0		0.0
Total assets	781.1		781.1
Liabilities [Abstract]
Accounts payable	2.1		2.1
Accrued expenses and other liabilities	59.2		59.2
Notes payable	109.8		109.8
Total liabilities	171.1		171.1
Revenues
Home sales	0.0		0.0
Land/lot sales and other	22.6		53.5
Financial services	0.0		0.0
Total revenues	22.6		53.5
Cost of sales
Home sales	0.0		0.0
Land/lot sales and other	16.2		35.5
Inventory and land option charges	0.0		0.0
Total cost of sales	16.2		35.5
Selling, General and Administrative Expense	5.6		19.1
Equity in earnings of unconsolidated entities	(1.5)		(9.1)
Gain on sale of assets	(2.7)		(2.7)
Interest Expense	2.1		4.2
Other (income) expense	(1.7)		(2.2)
Income (loss) before income taxes	4.6		8.7
Depreciation and amortization			2.5
Net cash (used in) provided by operating activities			(150.2)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	29.1		29.1		24.1
Restricted cash	6.0		6.0		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	658.2		658.2		587.3
Deferred income taxes	0.0		0.0		0.0
Property and equipment, net	3.0		3.0		3.0
Other Assets	42.0		42.0		42.2
Goodwill	0.0		0.0		0.0
Total assets	738.3		738.3		663.8
Liabilities [Abstract]
Accounts payable	3.0		3.0		1.5
Accrued expenses and other liabilities	35.6		35.6		35.6
Notes payable	489.8		489.8		420.0
Total liabilities	528.4		528.4		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	94.9	86.9	176.0	165.0
Total revenues	94.9	86.9	176.0	165.0
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	66.7	58.2	128.4	112.9
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	(3.2)	(3.5)	(6.1)	(6.7)
Income (loss) before income taxes	31.4	32.2	53.7	58.8
Depreciation and amortization			0.7	0.7
Net cash (used in) provided by operating activities			(30.7)	108.6
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	16.4		16.4		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	0.0		0.0		0.0
Property and equipment, net	170.8		170.8		127.6
Other Assets	3.9		3.9		5.0
Goodwill	0.0		0.0		0.0
Total assets	191.1		191.1		143.3
Liabilities [Abstract]
Accounts payable	8.7		8.7		3.3
Accrued expenses and other liabilities	15.2		15.2		16.3
Notes payable	0.0		0.0		0.0
Total liabilities	23.9		23.9		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	5.8	2.5	9.8	5.2
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	(3.6)	(1.8)	(6.5)	(2.2)
Income (loss) before income taxes	(2.2)	(0.7)	(3.3)	(3.0)
Depreciation and amortization			3.3	1.6
Net cash (used in) provided by operating activities			(0.5)	(3.9)
East [Member] | Homebuilding [Member]
Inventories:
Total inventories	1,188.2		1,188.2		1,068.9
Revenues
Total revenues	435.8	372.6	828.9	678.6
Cost of sales
Inventory and land option charges	0.7	6.2	0.6	6.5
Income (loss) before income taxes	46.7	25.9	91.7	52.2
Midwest [Member] | Homebuilding [Member]
Inventories:
Total inventories	545.6		545.6		492.6
Revenues
Total revenues	203.6	168.7	365.0	319.8
Cost of sales
Inventory and land option charges	0.2	0.2	0.4	0.3
Income (loss) before income taxes	18.7	0.7	32.0	10.9
Southeast [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,597.3		2,597.3		2,392.3
Revenues
Total revenues	1,042.0	969.0	2,030.6	1,852.4
Cost of sales
Inventory and land option charges	25.1	0.9	26.2	1.6
Income (loss) before income taxes	96.3	113.1	218.8	212.7
South Central [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,326.2		2,326.2		2,199.4
Revenues
Total revenues	919.0	815.4	1,727.8	1,572.3
Cost of sales
Inventory and land option charges	0.6	1.6	1.9	1.9
Income (loss) before income taxes	120.5	105.8	222.0	202.3
Southwest [Member] | Homebuilding [Member]
Inventories:
Total inventories	536.9		536.9		506.1
Revenues
Total revenues	172.1	126.7	328.5	235.2
Cost of sales
Inventory and land option charges	0.0	0.1	0.8	0.1
Income (loss) before income taxes	22.0	7.2	36.7	11.2
West [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,415.9		2,415.9		2,352.5
Revenues
Total revenues	913.2	712.0	1,625.8	1,332.2
Cost of sales
Inventory and land option charges	3.5	3.2	3.9	4.1
Income (loss) before income taxes	111.8	$ 69.7	188.6	$ 127.0
Corporate, Non-Segment [Member] | Homebuilding [Member]
Inventories:
Total inventories	$ 229.5		$ 229.5		$ 225.3