Notes Payable (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Unsecured:
Notes payable	$ 3,093.6		$ 2,871.6
Homebuilding [Member]
Unsecured:
Notes payable	2,447.1		2,451.6
Revolving credit facility, maturing 2022	0.0		0.0
Forestar Group [Member]
Unsecured:
Notes payable	120.2
Financial Services [Member]
Unsecured:
Notes payable	526.3		420.0
Forestar Group [Member]
Unsecured:
Notes payable	$ 110.5
3.625% senior notes due 2018 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	3.625%	3.625%
Notes payable	$ 0.0		399.7
3.75% senior notes due 2019 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	3.75%
Notes payable	$ 499.4		498.8
4.0% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.00%
Notes payable	$ 498.6		497.9
2.55% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	2.55%	2.55%
Notes payable	$ 397.8		0.0
4.375% senior notes due 2022 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.375%
Notes payable	$ 348.3		348.1
4.75% senior notes due 2023 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.75%
Notes payable	$ 298.6		298.4
5.75% senior notes due 2023 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	5.75%
Notes payable	$ 397.9		397.6
3.75% convertible senior notes due 2020 [Member] | Forestar Group [Member]
Unsecured:
Notes payable	$ 120.2
3.75% convertible senior notes due 2020 [Member] | Forestar Group [Member]
Unsecured:
Stated interest rate	3.75%
Secured Debt [Member] | Homebuilding [Member]
Unsecured:
Notes payable	$ 6.5		$ 11.1