Stockholders' Equity - Narrative (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
Number of shares repurchased during period	4.6	7.1
Cost of shares repurchased during period	$ 200.0	$ 261.7
Remaining number of shares authorized to be repurchased	27.0