Notes Payable - Narrative (Details)		3 Months Ended		6 Months Ended
	Aug. 01, 2017 USD ($)	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) $ / shares	Oct. 05, 2017 USD ($)	Sep. 30, 2017 USD ($)
Debt Instrument [Line Items]
Debt issuance costs		$ 10,100,000		$ 10,100,000		$ 9,500,000
Notes payable		3,233,900,000		3,233,900,000		2,871,600,000
Outstanding letters of credit		102,700,000		102,700,000
Restricted Cash and Cash Equivalents		55,400,000		55,400,000		16,500,000
Homebuilding [Member]
Debt Instrument [Line Items]
Notes payable		2,623,100,000		2,623,100,000		2,451,600,000
Borrowing capacity on revolving credit facility		1,275,000,000		1,275,000,000
Maximum borrowing capacity on revolving credit facility		$ 1,900,000,000		$ 1,900,000,000
Letters of credit, sublimit borrowing capacity, as a percentage		50.00%		50.00%
Proceeds from revolving credit facility				$ 1,500,000,000
Repayments of revolving credit facility				1,300,000,000
Borrowings outstanding on revolving credit facility		$ 175,000,000		$ 175,000,000		0
Line of Credit Facility, Interest Rate at Period End		3.30%		3.30%
Authorized repurchase of debt securities	$ 500,000,000
Debt repurchase program, remaining authorized repurchase amount		$ 500,000,000		$ 500,000,000
Restricted Cash and Cash Equivalents		9,400,000		9,400,000		9,300,000
Homebuilding [Member] | 3.625% senior notes due 2018 [Member]
Debt Instrument [Line Items]
Notes payable		$ 0		$ 0		399,700,000
Early Repayment of Senior Debt			$ 400,000,000
Stated interest rate		3.625%	3.625%	3.625%
Homebuilding [Member] | 2.55% senior notes due 2020 [Member]
Debt Instrument [Line Items]
Notes payable		$ 397,600,000		$ 397,600,000		0
Debt Instrument, Face Amount			$ 400,000,000
Debt Instrument, Interest Rate, Effective Percentage			2.80%
Stated interest rate		2.55%	2.55%	2.55%
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable		$ 489,800,000		$ 489,800,000		420,000,000
Borrowing capacity on revolving credit facility		600,000,000		600,000,000
Maximum borrowing capacity on revolving credit facility		1,000,000,000		1,000,000,000
Restricted Cash and Cash Equivalents		6,000,000		6,000,000		$ 7,200,000
Mortgage loans held for sale pledged under repurchase agreement		614,800,000		614,800,000
Mortgage loans held for sale pledged under repurchase agreement, collateral value		595,500,000		595,500,000
Advance pay downs on mortgage repurchase facility		$ 105,700,000		$ 105,700,000
Interest rate on mortgage repurchase facility		3.70%		3.70%
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable		$ 109,800,000		$ 109,800,000
Borrowing capacity on revolving credit facility		30,000,000		30,000,000	$ 50,000,000
Letter of Credit, Maximum Borrowing Capacity (in dollars)					50,000,000
Outstanding letters of credit		21,000,000		21,000,000	14,300,000
Restricted Cash and Cash Equivalents		$ 40,000,000		$ 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		$ 579.77062
Stated interest rate		3.75%		3.75%	3.75%
Line of Credit [Member] | Homebuilding [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		$ 79,200,000		$ 79,200,000
Weighted Average [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Borrowing capacity on revolving credit facility		725,000,000		725,000,000
Maximum [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Borrowing capacity on revolving credit facility		800,000,000		800,000,000
Segment Reconciling Items [Member]
Debt Instrument [Line Items]
Notes payable		11,200,000		11,200,000
Restricted Cash and Cash Equivalents		$ 0		$ 0