Investment in Unconsolidated Entities - Condensed Financial Information of Unconsolidated Entities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Assets:
Assets	$ 96.5		$ 96.5
Liabilities and Equity:
Debt	45.8		45.8
Equity	45.5		45.5
Liabilities and Equity	96.5		96.5
Revenues	3.4		12.1
Net earnings of unconsolidated entities (1)	4.3		21.7
Equity in earnings of unconsolidated entities	0.4	$ 0.0	2.7	$ 0.0
Forestar Group [Member]
Liabilities and Equity:
Equity in earnings of unconsolidated entities	1.5		9.1
Accounts Payable [Member]
Liabilities and Equity:
Accounts payable and other liabilities	5.2		5.2
Cash and Cash Equivalents [Member]
Assets:
Current assets	6.4		6.4
Inventories [Member]
Assets:
Current assets	88.8		88.8
Other Assets [Member]
Assets:
Current assets	$ 1.3		$ 1.3