Earnings Per Share (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Number of shares excluded from computation of earnings per share									8.5	4.6	6.4
Earnings Per Share Reconciliation [Abstract]
Net income	$ 166.3	$ 113.1	$ 131.0	$ 123.2	$ 139.5	$ 146.0	$ 111.0	$ 66.3	$ 533.5	$ 462.7	$ 956.3
Interest expense and amortization of issuance costs associated with convertible senior notes									16.5	23.9	36.8
Numerator for diluted earnings per share after assumed conversions									$ 550.0	$ 486.6	$ 993.1
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Denominator for basic earnings per share - weighted average common shares									340.5	322.1	318.1
Employee stock awards									3.1	4.2	2.6
Convertible senior notes									23.0	38.6	38.3
Denominator for diluted earnings per share - adjusted weighted average common shares									366.6	364.9	359.0