Stockholders' Equity Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	9 Months Ended		86 Months Ended
	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 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	1.7	9.5	7.8	73.8
Stock Repurchased During Period, Value	$ 29.6	$ 169.2	$ 120.9	$ 1,238.8
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 261.2		$ 261.2