Stockholders' Equity Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	6 Months Ended		59 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2018	Aug. 01, 2018	Apr. 29, 2015	Apr. 29, 2013
Stockholders Equity Note [Line Items]
Document Period End Date	Mar. 31, 2019
Stock Repurchase Program, Number of Shares Authorized to be Repurchased, Value				$ 500.0	$ 500.0	$ 500.0
Stock Repurchased and Retired During Period, Shares	6.1	5.8	62.2
Stock Repurchased During Period, Value	$ 91.3	$ 99.1	$ 1,034.0
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 466.0