Notes Payable (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Unsecured:
Notes payable	$ 3,258.1	$ 2,871.6
Homebuilding [Member]
Unsecured:
Notes payable	2,749.6	2,451.6
Revolving credit facility, maturing 2022	300.0	0.0
Forestar Group [Member]
Unsecured:
Notes payable	121.0
Financial Services [Member]
Unsecured:
Notes payable	387.5	420.0
Forestar Group [Member]
Unsecured:
Notes payable	$ 108.4
3.625% senior notes due 2018 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	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	$ 499.0	498.8
4.0% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.00%
Notes payable	$ 498.1	497.9
2.55% senior notes due 2020 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	2.55%
Notes payable	$ 397.4	0.0
4.375% senior notes due 2022 [Member] | Homebuilding [Member]
Unsecured:
Stated interest rate	4.375%
Notes payable	$ 348.1	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	$ 397.7	397.6
3.75% convertible senior notes due 2020 [Member] | Forestar Group [Member]
Unsecured:
Notes payable	$ 120.7
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	$ 10.8	$ 11.1
Secured Debt [Member] | Forestar Group [Member]
Unsecured:
Notes payable	$ 0.3