Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Income tax expense	$ 105.4	$ 82.2	$ 189.0	$ 160.4
Effective tax rate (percent)	35.10%	35.70%	34.90%	35.60%
Deferred tax assets net of DTL	$ 536.7		$ 536.7		$ 568.2
Valuation allowance for deferred income taxes	$ 10.1		$ 10.1		$ 10.1