Notes Payable - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended
	Aug. 01, 2018	Sep. 30, 2019	Mar. 31, 2019	Jun. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Document Period End Date				Jun. 30, 2019
Notes payable				$ 3,450,600,000	$ 3,203,500,000
Outstanding letters of credit				162,300,000
Restricted Cash and Cash Equivalents				21,100,000	32,900,000
Home Building [Member]
Debt Instrument [Line Items]
Debt issuance costs				6,100,000	8,500,000
Notes payable				2,191,300,000	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				$ 2,100,000,000
Repayments of revolving credit facility				1,950,000,000
Borrowings outstanding on revolving credit facility				$ 150,000,000.0	0
Line of Credit Facility, Interest Rate at Period End				3.60%
Authorized repurchase of debt securities	$ 500,000,000
Debt repurchase program, remaining authorized repurchase amount				$ 500,000,000
Restricted Cash and Cash Equivalents				9,800,000	8,600,000
Home Building [Member] | Senior Note Twenty Nine [Member]
Debt Instrument [Line Items]
Notes payable				$ 0	499,600,000
Maturities of Senior Debt			$ 500,000,000
Stated interest rate				3.75%
Financial Services [Member]
Debt Instrument [Line Items]
Notes payable				$ 796,500,000	637,700,000
Borrowing capacity on revolving credit facility				900,000,000
Maximum borrowing capacity on revolving credit facility				1,200,000,000
Mortgage loans held for sale pledged under repurchase agreement				906,200,000
Mortgage loans held for sale pledged under repurchase agreement, collateral value				$ 874,700,000
Interest rate on mortgage repurchase facility				4.10%
Forestar Group [Member]
Debt Instrument [Line Items]
Notes payable				$ 458,900,000	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%
Restricted Cash and Cash Equivalents				$ 200,000	16,200,000
Forestar Group [Member] | 3.75% convertible senior notes due 2020 [Member]
Debt Instrument [Line Items]
Stated interest rate				3.75%
Forestar Group [Member] | Senior Note Member Forty One [Member]
Debt Instrument [Line Items]
Debt issuance costs				$ 6,500,000
Stated interest rate				8.00%
Debt Instrument, Interest Rate, Effective Percentage				8.50%
Debt Instrument, Face Amount				$ 350,000,000
Forestar Consolidated [Member]
Debt Instrument [Line Items]
Notes payable				462,800,000	119,900,000
Proceeds from revolving credit facility				85,000,000
Repayments of revolving credit facility				85,000,000
Borrowings outstanding on revolving credit facility				0	0
Forestar Consolidated [Member] | 3.75% convertible senior notes due 2020 [Member]
Debt Instrument [Line Items]
Notes payable				119,300,000	119,900,000
Forestar Consolidated [Member] | Senior Note Member Forty One [Member]
Debt Instrument [Line Items]
Notes payable				343,500,000	0
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Outstanding letters of credit				1,700,000
Standby Letters of Credit [Member] | Home Building [Member]
Debt Instrument [Line Items]
Outstanding letters of credit				1,500,000
Standby Letters of Credit [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Outstanding letters of credit				200,000
Line of Credit [Member] | Home Building [Member]
Debt Instrument [Line Items]
Outstanding letters of credit				138,900,000
Line of Credit [Member] | Forestar Group [Member]
Debt Instrument [Line Items]
Outstanding letters of credit				21,700,000
Maximum [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Borrowing capacity on revolving credit facility				1,100,000,000
Segment Reconciling Items [Member]
Debt Instrument [Line Items]
Notes payable				3,900,000	8,200,000
Restricted Cash and Cash Equivalents				$ 0	$ 0
Subsequent Event [Member] | Home Building [Member]
Debt Instrument [Line Items]
Authorized repurchase of debt securities		$ 500,000,000