Notes Payable - Narrative (Details)		3 Months Ended
	Aug. 01, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($)
Debt Instrument [Line Items]
Debt issuance costs		$ 11,000,000	$ 9,500,000
Notes payable		3,258,100,000	2,871,600,000
Outstanding letters of credit		114,900,000
Restricted Cash and Cash Equivalents		53,700,000	16,500,000
Homebuilding [Member]
Debt Instrument [Line Items]
Notes payable		2,749,600,000	2,451,600,000
Borrowing capacity on revolving credit facility		1,275,000,000
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		$ 715,000,000
Repayments of revolving credit facility		415,000,000
Borrowings outstanding		$ 300,000,000	0
Line of Credit Facility, Interest Rate at Period End		3.40%
Authorized repurchase of debt securities	$ 500,000,000
Debt repurchase program, remaining authorized repurchase amount		$ 500,000,000
Restricted Cash and Cash Equivalents		8,300,000	9,300,000
Homebuilding [Member] | 3.625% senior notes due 2018 [Member]
Debt Instrument [Line Items]
Notes payable		0	399,700,000
Early Repayment of Senior Debt		$ 400,000,000
Stated interest rate		3.625%
Homebuilding [Member] | 2.55% senior notes due 2020 [Member]
Debt Instrument [Line Items]
Notes payable		$ 397,400,000	0
Debt Instrument, Face Amount		$ 400,000,000
Debt Instrument, Interest Rate, Effective Percentage		2.80%
Stated interest rate		2.55%
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable		$ 387,500,000	420,000,000
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		5,400,000	7,200,000
Mortgage loans held for sale pledged under repurchase agreement		494,600,000
Mortgage loans held for sale pledged under repurchase agreement, collateral value		476,600,000
Advance pay downs on mortgage repurchase facility		$ 89,100,000
Interest rate on mortgage repurchase facility		3.70%
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable		$ 108,400,000
Borrowing capacity on revolving credit facility		30,000,000	50,000,000
Letter of Credit, Maximum Borrowing Capacity (in dollars)			50,000,000
Outstanding letters of credit		14,400,000	$ 14,300,000
Restricted Cash and Cash Equivalents		40,000,000
Forestar Group [Member] | 3.75% convertible senior notes due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		120,000,000
Early Repayment of Senior Debt		$ 1,100,000
Debt Instrument, Convertible, Conversion Ratio		0.00817192	0.0408351
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 579.77062
Stated interest rate		3.75%
Forestar Group [Member] | Senior Note Member Thirty Nine [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 5,300,000
Early Repayment of Senior Debt		$ 5,900,000
Stated interest rate		8.50%
Line of Credit [Member] | Homebuilding [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		$ 98,000,000
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		12,600,000
Restricted Cash and Cash Equivalents		$ 0