Earnings Per Share Computations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Numerator
Net income	$ 54.4us-gaap_NetIncomeLoss	$ 49.2us-gaap_NetIncomeLoss
Denominator
Basic weighted-average common shares outstanding	97.7	101.5
Dilutive shares	1.1	0.9
Weighted-average common and common equivalent shares outstanding - diluted	98.8	102.4
Per common share income
Net income, per basic share	$ 0.55us-gaap_EarningsPerShareBasic	$ 0.48us-gaap_EarningsPerShareBasic
Net income, per diluted share	$ 0.55us-gaap_EarningsPerShareDiluted	$ 0.48us-gaap_EarningsPerShareDiluted
Antidilutive stock options and stock awards	0	0.4

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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