Notes Payable - Narrative (Details) - USD ($)		3 Months Ended
	Aug. 01, 2018	Dec. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Debt issuance costs		$ 7,600,000	$ 8,500,000
Notes payable		3,342,300,000	3,203,500,000
Outstanding letters of credit		144,500,000
Restricted Cash and Cash Equivalents		31,300,000	32,900,000
Homebuilding [Member]
Debt Instrument [Line Items]
Notes payable		2,748,700,000	2,445,900,000
Borrowing capacity on revolving credit facility		1,325,000,000
Maximum borrowing capacity on revolving credit facility		$ 1,900,000,000
Letters of credit, sublimit borrowing capacity, as a percentage		50.00%
Proceeds from revolving credit facility		$ 575,000,000
Repayments of revolving credit facility		275,000,000
Borrowings outstanding on revolving credit facility		$ 300,000,000	0
Line of Credit Facility, Interest Rate at Period End		4.10%
Authorized repurchase of debt securities	$ 500,000,000
Debt repurchase program, remaining authorized repurchase amount		$ 500,000,000
Restricted Cash and Cash Equivalents		8,700,000	8,600,000
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable		473,900,000	637,700,000
Borrowing capacity on revolving credit facility		600,000,000
Maximum borrowing capacity on revolving credit facility		1,000,000,000
Restricted Cash and Cash Equivalents		6,500,000	8,100,000
Mortgage loans held for sale pledged under repurchase agreement		573,800,000
Mortgage loans held for sale pledged under repurchase agreement, collateral value		$ 555,600,000
Interest rate on mortgage repurchase facility		4.40%
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable		$ 112,900,000	111,700,000
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%
Borrowings outstanding on revolving credit facility		$ 0	0
Restricted Cash and Cash Equivalents		$ 16,100,000	16,200,000
Forestar Group [Member] | 3.75% convertible senior notes due 2020 [Member]
Debt Instrument [Line Items]
Stated interest rate		3.75%
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		$ 16,700,000
Standby Letters of Credit [Member] | Homebuilding [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		1,400,000
Standby Letters of Credit [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		15,300,000
Line of Credit [Member] | Homebuilding [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		124,400,000
Line of Credit [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		3,400,000
Weighted Average [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Borrowing capacity on revolving credit facility		725,000,000
Maximum [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Borrowing capacity on revolving credit facility		800,000,000
Segment Reconciling Items [Member]
Debt Instrument [Line Items]
Notes payable		6,800,000	8,200,000
Restricted Cash and Cash Equivalents		$ 0	$ 0