Notes Payable (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Unsecured:
Notes payable	$ 3,342.3	$ 3,203.5
Homebuilding [Member]
Unsecured:
Notes payable	2,748.7	2,445.9
Revolving credit facility	300.0	0.0
Forestar Group [Member]
Unsecured:
Notes payable	112.9	111.7
Revolving credit facility	0.0	0.0
Financial Services [Member]
Unsecured:
Notes payable	473.9	637.7
Forestar Group [Member]
Unsecured:
Notes payable	$ 119.7	119.9
3.75% senior notes due 2019 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	3.75%
Notes payable	$ 499.9	499.6
4.0% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.00%
Notes payable	$ 499.0	498.8
2.55% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	2.55%
Notes payable	$ 398.2	397.9
4.375% senior notes due 2022 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.375%
Notes payable	$ 348.5	348.4
4.75% senior notes due 2023 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.75%
Notes payable	$ 298.7	298.7
5.75% senior notes due 2023 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	5.75%
Notes payable	$ 398.1	398.0
3.75% convertible senior notes due 2020 [Member] | Forestar Group [Member]
Unsecured:
Stated interest rate	3.75%
3.75% convertible senior notes due 2020 [Member] | Forestar Group [Member]
Unsecured:
Notes payable	$ 119.7	119.9
Secured Debt [Member] | Homebuilding [Member]
Unsecured:
Notes payable	$ 6.3	$ 4.5