Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Deferred tax assets:
Inventory costs	$ 40.9	$ 42.6
Inventory impairments	31.8	83.9
Warranty and construction defect costs	121.8	163.7
Net operating loss carryforwards	38.1	26.2
Tax credit carryforwards	4.3	2.5
Incentive compensation plans	55.2	92.6
Deferred income	1.3	1.7
Other	5.8	13.9
Total deferred tax assets	299.2	427.1
Valuation allowance	(17.7)	(11.2)
Total deferred tax assets, net of valuation allowance	281.5	415.9
Deferred tax liabilities:
Deferral of profit on home sales	64.9	41.6
Other	22.6	9.3
Total deferred tax liabilities	87.5	50.9
Deferred income taxes, net	$ 194.0	$ 365.0