Commitments and Contingencies (Details Textuals) - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Loss Contingency [Abstract]
Liabilities for various claims, complaints and other legal actions	$ 434,700,000	$ 408,100,000	$ 420,600,000
Construction defect portion of loss contingency accrual	99.00%	99.00%
Expenses related to legal claims	$ 15,300,000	$ 41,000,000.0	87,800,000
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	75,100,000	54,600,000
Other Commitments [Abstract]
Surety bonds	1,700,000,000
Outstanding letters of credit	171,700,000
Rent expense	30,500,000	27,800,000	$ 26,300,000
Letter of credit, other [Member]
Other Commitments [Abstract]
Outstanding letters of credit	800,000
Home Building [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	515,400,000
Purchase Obligation	7,200,000,000
Home Building [Member] | Revolving credit facility [Member]
Other Commitments [Abstract]
Outstanding letters of credit	141,200,000
Home Building [Member] | Cash Deposits [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	496,000,000.0
Home Building [Member] | Promissory notes and letters of credit [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	19,400,000
Forestar Consolidated [Member]
Land and Lot Option Purchase Contracts [Abstract]
Deposits	88,700,000
Purchase Obligation	953,800,000
Forestar Group [Member]
Land and Lot Option Purchase Contracts [Abstract]
Increase (Decrease) in Earnest Money Deposits Outstanding	34,500,000	21,200,000
Increase (Decrease) in Prepaid Expenses, Other	13,100,000	$ 15,200,000
Forestar Group [Member] | Revolving credit facility [Member]
Other Commitments [Abstract]
Outstanding letters of credit	29,700,000
Individual Claim Limit [Member]
Loss Contingency [Abstract]
Self-insurance limits	250,000
Option Contracts Subject to Specific Performance Clauses [Member] | Home Building [Member]
Land and Lot Option Purchase Contracts [Abstract]
Purchase Obligation	59,800,000
Option Contracts Subject to Specific Performance Clauses [Member] | Forestar Consolidated [Member]
Land and Lot Option Purchase Contracts [Abstract]
Purchase Obligation	13,200,000
Minimum [Member]
Loss Contingency [Abstract]
Amount of self insurance	10,000,000
Maximum [Member]
Loss Contingency [Abstract]
Amount of self insurance	20,000,000
Future Policy Year [Member]
Loss Contingency [Abstract]
Reinsurance coverage acquired by captive insurance subsidiary	5,000,000
Captive Insurance - Individual claim limit beyond reinsurance coverage	$ 20,000,000