Notes Payable (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Oct. 05, 2017	Sep. 30, 2017
Unsecured:
Notes payable	$ 3,233.9			$ 2,871.6
Homebuilding [Member]
Unsecured:
Notes payable	2,623.1			2,451.6
Revolving credit facility, maturing 2022	175.0			0.0
Forestar Group [Member]
Unsecured:
Notes payable	121.0
Financial Services [Member]
Unsecured:
Notes payable	489.8			420.0
Forestar Group [Member]
Unsecured:
Notes payable	$ 109.8
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	$ 499.2			498.8
4.0% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.00%
Notes payable	$ 498.4			497.9
2.55% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	2.55%	2.55%
Notes payable	$ 397.6			0.0
4.375% senior notes due 2022 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.375%
Notes payable	$ 348.2			348.1
4.75% senior notes due 2023 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.75%
Notes payable	$ 298.5			298.4
5.75% senior notes due 2023 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	5.75%
Notes payable	$ 397.8			397.6
3.75% convertible senior notes due 2020 [Member] | Forestar Group [Member]
Unsecured:
Notes payable	$ 120.5
3.75% convertible senior notes due 2020 [Member] | Forestar Group [Member]
Unsecured:
Stated interest rate	3.75%		3.75%
Secured Debt [Member] | Homebuilding [Member]
Unsecured:
Notes payable	$ 8.4			$ 11.1
Secured Debt [Member] | Forestar Group [Member]
Unsecured:
Notes payable	$ 0.5