Commitments and Contingencies (Details Textuals) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Loss Contingency [Abstract]
Liabilities for various claims, complaints and other legal actions	$ 408,100,000	$ 420,600,000	$ 423,500,000
Construction defect portion of loss contingency accrual	99.00%	98.00%
Expenses related to legal claims	$ 41,000,000	$ 87,800,000	49,600,000
Amount of Captive Insurance - Aggregate Limit	25,000,000
Amount of Captive Insurance - Individual Claim Limit	250,000
Reinsurance coverage acquired by captive insurance subsidiary	15,000,000
Captive Insurance - Aggregate amount beyond reinsurance coverage	10,000,000
Captive Insurance - Individual claim limit beyond reinsurance coverage	250,000
Maximum amount deductible under workers compensation insurance policy	500,000
Deductible policy amount per occurrence	500,000
Estimated insurance recoveries related to legal claims	54,600,000	74,400,000
Other Commitments [Abstract]
Surety bonds	1,500,000,000
Outstanding letters of credit	128,500,000
Rent expense	27,800,000	$ 26,300,000	$ 24,700,000
Letter of credit, other [Member]
Other Commitments [Abstract]
Outstanding letters of credit	16,800,000
Homebuilding [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	401,100,000
Purchase Obligation	6,500,000,000
Homebuilding [Member] | Revolving credit facility [Member]
Other Commitments [Abstract]
Outstanding letters of credit	107,200,000
Homebuilding [Member] | Letter of credit, other [Member]
Other Commitments [Abstract]
Outstanding letters of credit	1,400,000
Homebuilding [Member] | Cash Deposits [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	397,400,000
Homebuilding [Member] | Promissory notes and letters of credit [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	3,700,000
Forestar Consolidated [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	48,000,000
Purchase Obligation	522,000,000
Forestar Group [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	4,200,000
Purchase Obligation	32,700,000
Increase (Decrease) in Earnest Money Deposits Outstanding	21,200,000
Increase (Decrease) in Prepaid Expenses, Other	15,200,000
Forestar Group [Member] | Revolving credit facility [Member]
Other Commitments [Abstract]
Outstanding letters of credit	4,500,000
Forestar Group [Member] | Letter of credit, other [Member]
Other Commitments [Abstract]
Outstanding letters of credit	15,400,000
Individual Claim Limit [Member]
Loss Contingency [Abstract]
Self-insurance limits	250,000.00
Option Contracts Subject to Specific Performance Clauses [Member] | Homebuilding [Member]
Land and Lot Option Purchase Contracts [Abstract]
Purchase Obligation	82,100,000
Option Contracts Subject to Specific Performance Clauses [Member] | Forestar Consolidated [Member]
Land and Lot Option Purchase Contracts [Abstract]
Purchase Obligation	34,700,000
Minimum [Member]
Loss Contingency [Abstract]
Amount of self insurance	10,000,000
Maximum [Member]
Loss Contingency [Abstract]
Amount of self insurance	$ 17,500,000