Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest [Abstract]
Number of shares excluded from computation of earnings per share									7.3	8.5	4.6
Earnings Per Share Reconciliation [Abstract]
Net income	$ 238.9	$ 221.4	$ 147.9	$ 142.5	$ 166.3	$ 113.1	$ 131.0	$ 123.2	$ 750.7	$ 533.5	$ 462.7
Interest expense and amortization of issuance costs associated with convertible senior notes									0.0	16.5	23.9
Numerator for diluted earnings per share after assumed conversions									$ 750.7	$ 550.0	$ 486.6
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Denominator for basic earnings per share - weighted average common shares									366.3	340.5	322.1
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
Employee stock awards									3.5	3.1	4.2
Convertible senior notes									0.0	23.0	38.6
Denominator for diluted earnings per share - adjusted weighted average common shares									369.8	366.6	364.9
Earnings Per Share, Basic	$ 0.65	$ 0.60	$ 0.40	$ 0.39	$ 0.46	$ 0.32	$ 0.40	$ 0.38	$ 2.05	$ 1.57	$ 1.44
Earnings Per Share, Diluted	$ 0.64	$ 0.60	$ 0.40	$ 0.39	$ 0.45	$ 0.32	$ 0.38	$ 0.36	$ 2.03	$ 1.50	$ 1.33