Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Income tax expense	$ 124.7	$ 105.4	$ 236.0	$ 189.0
Effective tax rate (percent)	35.20%	35.10%	35.10%	34.90%
Deferred tax assets net of DTL	$ 461.9		$ 461.9		$ 486.6
Homebuilding [Member]
Income Tax Contingency [Line Items]
Valuation allowance for deferred income taxes	$ 10.3		$ 10.3		$ 10.3