Shareholders' Equity (Details) - USD ($) $ in Millions	12 Months Ended		25 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2017	Aug. 31, 2015
Stockholders' Equity Note [Abstract]
Number of shares of common stock authorized for buyback (in shares)				10,000,000
Remaining number of shares authorized to be repurchased (in shares)			7,512,574	10,299,198
Shares repurchased under authorization (in shares)	2,500,000.0	4,900,000.0	2,786,624
Aggregate cost of common stock repurchased	$ 100.1	$ 200.4	$ 112.0