Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Number of shares excluded from computation of earnings per share										7.3	8.5
Numerator:
Net income	$ 283.6	$ 249.8	$ 195.1	$ 157.7	$ 238.9	$ 221.4	$ 147.9	$ 142.5	$ 886.3	$ 750.7	$ 533.5
Interest expense and amortization of issuance costs associated with convertible senior notes									0.0	0.0	16.5
Numerator for diluted earnings per share after assumed conversions									$ 886.3	$ 750.7	$ 550.0
Denominator:
Denominator for basic earnings per share - weighted average common shares									371.0	366.3	340.5
Effect of dilutive securities:
Employee stock awards									4.1	3.5	3.1
Convertible senior notes									0.0	0.0	23.0
Denominator for diluted earnings per share - adjusted weighted average common shares									375.1	369.8	366.6
Basic net income per common share	$ 0.76	$ 0.67	$ 0.53	$ 0.43	$ 0.65	$ 0.60	$ 0.40	$ 0.39	$ 2.39	$ 2.05	$ 1.57
Net income per common share assuming dilution	$ 0.75	$ 0.66	$ 0.52	$ 0.42	$ 0.64	$ 0.60	$ 0.40	$ 0.39	$ 2.36	$ 2.03	$ 1.50