Stockholders' Equity Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		59 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2018	Aug. 01, 2018	Apr. 29, 2015	Apr. 29, 2013
Stockholders Equity Note [Line Items]
Document Period End Date	Dec. 31, 2018
Stock Repurchase Program, Number of Shares Authorized to be Repurchased, Value				$ 500.0	$ 500.0	$ 500.0
Stock Repurchased and Retired During Period, Shares	4.9	5.8	61.0
Stock Repurchased During Period, Value	$ 75.2	$ 99.1	$ 1,017.8
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 482.2