Earnings Per Share (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Net income	$ 111.0	$ 40.6	$ 177.3	$ 68.3
Interest expense and amortization of issuance costs associated with convertible senior notes	5.8	0	11.5	0
Numerator for diluted earnings per share after assumed conversions	$ 116.8	$ 40.6	$ 188.8	$ 68.3
Denominator:
Denominator for basic earnings per share - weighted average common shares	321.7	317.6	321.4	317.0
Effect of dilutive securities:
Employee stock awards	5.1	2.5	4.7	1.3
Incremental Common Shares Attributable to Conversion of Debt Securities	38.6	0	38.6	0
Denominator for diluted earnings per share - adjusted weighted average common shares	365.4	320.1	364.7	318.3
Earnings (Loss) Per Share [Abstract]
Number of shares excluded from computation of earnings per share			5.6	8.7