Stockholder's Equity Share Repurchase (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended	6 Months Ended
	Jan. 17, 2017	Mar. 31, 2017	Mar. 31, 2017
Equity, Class of Treasury Stock [Line Items]
Authorized amount of stock for repurchase	$ 500.0
2017 Stock Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Share repurchase authorized date	Jan. 17, 2017
Stock repurchase program expiration date	Jan. 17, 2019
Stock repurchased		$ 95.2	$ 201.7
Stock repurchased (shares)		1.0	2.4
Average price of stock repurchased (in dollars per share)		$ 95.23	$ 84.04
Remaining amount authorized fro stock repurchase		$ 404.8	$ 404.8