Notes Payable - Narrative (Details) - USD ($)		3 Months Ended	6 Months Ended
	Aug. 01, 2018	Mar. 31, 2019	Mar. 31, 2019	Jun. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Debt issuance costs		$ 6,800,000	$ 6,800,000		$ 8,500,000
Notes payable		3,622,400,000	3,622,400,000		3,203,500,000
Outstanding letters of credit		148,300,000	148,300,000
Restricted Cash and Cash Equivalents		32,400,000	32,400,000		32,900,000
Homebuilding [Member]
Debt Instrument [Line Items]
Notes payable		2,777,200,000	2,777,200,000		2,445,900,000
Borrowing capacity on revolving credit facility		1,325,000,000	1,325,000,000
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			$ 1,800,000,000
Repayments of revolving credit facility			1,000,000,000
Borrowings outstanding on revolving credit facility		$ 750,000,000	$ 750,000,000		0
Line of Credit Facility, Interest Rate at Period End		3.70%	3.70%
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		8,600,000
Homebuilding [Member] | Senior Note Twenty Nine [Member]
Debt Instrument [Line Items]
Notes payable		0	$ 0		499,600,000
Maturities of Senior Debt		$ 500,000,000
Stated interest rate		3.75%	3.75%
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable		$ 690,700,000	$ 690,700,000		637,700,000
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		7,300,000	7,300,000		8,100,000
Mortgage loans held for sale pledged under repurchase agreement		784,500,000	784,500,000
Mortgage loans held for sale pledged under repurchase agreement, collateral value		$ 759,400,000	$ 759,400,000
Interest rate on mortgage repurchase facility		4.20%	4.20%
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable		$ 149,200,000	$ 149,200,000		111,700,000
Borrowing capacity on revolving credit facility		380,000,000	380,000,000
Letter of Credit, Maximum Borrowing Capacity (in dollars)		100,000,000	100,000,000
Maximum borrowing capacity on revolving credit facility		$ 570,000,000	$ 570,000,000
Letters of credit, sublimit borrowing capacity, as a percentage		50.00%	50.00%
Line of Credit Facility, Interest Rate at Period End		4.80%	4.80%
Restricted Cash and Cash Equivalents		$ 15,700,000	$ 15,700,000		16,200,000
Forestar Group [Member] | 3.75% convertible senior notes due 2020 [Member]
Debt Instrument [Line Items]
Stated interest rate		3.75%	3.75%
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		$ 16,500,000	$ 16,500,000
Standby Letters of Credit [Member] | Homebuilding [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		1,500,000	1,500,000
Standby Letters of Credit [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		15,000,000	15,000,000
Line of Credit [Member] | Homebuilding [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		128,000,000	128,000,000
Line of Credit [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		3,800,000	3,800,000
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		5,300,000	5,300,000		8,200,000
Restricted Cash and Cash Equivalents		$ 0	$ 0		$ 0
Subsequent Event [Member] | Forestar Group [Member] | Senior Note Member Forty One [Member]
Debt Instrument [Line Items]
Stated interest rate				8.00%
Debt Instrument, Interest Rate, Effective Percentage				8.50%
Debt Instrument, Face Amount				$ 350,000,000