Notes Payable - Narrative (Details) - USD ($)	3 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Oct. 01, 2019
Debt Instrument [Line Items]
Notes payable	$ 3,783,300,000	$ 3,399,400,000
Outstanding letters of credit	170,400,000
Restricted Cash and Cash Equivalents	12,600,000	19,700,000
Home Building [Member]
Debt Instrument [Line Items]
Repayments of Long-term Lines of Credit	0
Unamortized Debt Issuance Expense	8,600,000	5,400,000
Notes payable	2,470,000,000.0	2,047,600,000
Borrowing capacity on revolving credit facility	1,590,000,000
Maximum borrowing capacity on revolving credit facility	$ 2,500,000,000
Letters of credit, sublimit borrowing capacity, as a percentage	100.00%
Proceeds from revolving credit facility	$ 0
Borrowings outstanding on revolving credit facility	0	0
Line of Credit Facility, Remaining Borrowing Capacity	1,400,000,000
Authorized repurchase of debt securities		500,000,000
Restricted Cash and Cash Equivalents	6,600,000	8,000,000.0
Home Building [Member] | 5.75% senior notes due 2023 [Member]
Debt Instrument [Line Items]
Notes payable	$ 398,400,000	398,400,000
Stated interest rate	5.75%
Home Building [Member] | SeniorNoteFortyTwo [Member]
Debt Instrument [Line Items]
Notes payable	$ 495,800,000	0	$ 500,000,000
Stated interest rate	2.50%		2.50%
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable	$ 850,200,000	888,900,000
Borrowing capacity on revolving credit facility	900,000,000
Maximum borrowing capacity on revolving credit facility	1,200,000,000
Temporary Increase In Borrowing Capacity	50,000,000
Restricted Cash and Cash Equivalents	5,900,000	11,600,000
Mortgage loans held for sale pledged under repurchase agreement	953,700,000
Mortgage loans held for sale pledged under repurchase agreement, collateral value	$ 915,100,000
Interest rate on mortgage repurchase facility	3.50%
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	$ 462,100,000	460,500,000
Borrowing capacity on revolving credit facility	380,000,000
Letter of Credit, Maximum Borrowing Capacity (in dollars)	100,000,000
Maximum borrowing capacity on revolving credit facility	$ 570,000,000
Letters of credit, sublimit borrowing capacity, as a percentage	50.00%
Line of Credit Facility, Remaining Borrowing Capacity	$ 351,300,000
Restricted Cash and Cash Equivalents	$ 0	0
Forestar Group [Member] | 3.75% convertible senior notes due 2020 [Member]
Debt Instrument [Line Items]
Stated interest rate	3.75%
Debt Instrument, Face Amount	$ 118,900,000
Forestar Group [Member] | Senior Note Member Forty One [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	$ 5,900,000	6,200,000
Stated interest rate	8.00%
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable	$ 463,100,000	462,900,000
Borrowings outstanding on revolving credit facility	0	0
Forestar Group [Member] | 3.75% convertible senior notes due 2020 [Member]
Debt Instrument [Line Items]
Notes payable	119,000,000.0	119,100,000
Forestar Group [Member] | Senior Note Member Forty One [Member]
Debt Instrument [Line Items]
Notes payable	344,100,000	343,800,000
Line of Credit [Member] | Home Building [Member]
Debt Instrument [Line Items]
Outstanding letters of credit	141,700,000
Line of Credit [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Outstanding letters of credit	28,700,000
Maximum [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Borrowing capacity on revolving credit facility	950,000,000
Segment Reconciling Items [Member]
Debt Instrument [Line Items]
Notes payable	1,000,000.0	2,400,000
Restricted Cash and Cash Equivalents	$ 0	$ 0