Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Number of shares excluded from computation of earnings per share			8.7	9.1
Earnings Per Share Reconciliation [Abstract]
Net income	$ 147.9us-gaap_ProfitLoss	$ 131.0us-gaap_ProfitLoss	$ 290.4us-gaap_ProfitLoss	$ 254.1us-gaap_ProfitLoss
Interest expense and amortization of issuance costs associated with convertible senior notes	0	6.9	0	13.7
Numerator for diluted earnings per share after assumed conversions	$ 147.9	$ 137.9	$ 290.4	$ 267.8
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Denominator for basic earnings per share - weighted average common shares	365.8	324.3	365.4	323.7
Employee stock awards	3.6	3.4	3.4	3.1
Incremental Common Shares Attributable to Conversion of Debt Securities	0	38.6	0	38.6
Denominator for diluted earnings per share - adjusted weighted average common shares	369.4	366.3	368.8	365.4
Earnings Per Share, Basic	$ 0.40us-gaap_EarningsPerShareBasic	$ 0.40us-gaap_EarningsPerShareBasic	$ 0.79us-gaap_EarningsPerShareBasic	$ 0.79us-gaap_EarningsPerShareBasic
Earnings Per Share, Diluted	$ 0.40us-gaap_EarningsPerShareDiluted	$ 0.38us-gaap_EarningsPerShareDiluted	$ 0.79us-gaap_EarningsPerShareDiluted	$ 0.73us-gaap_EarningsPerShareDiluted

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible debt securities using the if-converted method.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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