Stockholders' Equity Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended		83 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Aug. 01, 2018	Apr. 29, 2015	Apr. 29, 2013
Stockholders Equity Note [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased, Value						$ 500.0	$ 500.0	$ 500.0
Stock Repurchased and Retired During Period, Shares	3.8	1.2	9.5	6.1	73.8
Stock Repurchased During Period, Value	$ 76.8	$ 16.2	$ 169.2	$ 91.3	$ 1,238.8
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 261.2		$ 261.2		$ 261.2