Net Income Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Basic net income per share:
Net income	$ 28.4us-gaap_NetIncomeLoss	$ 25.4us-gaap_NetIncomeLoss
Weighted average shares outstanding:
Common stock	141.4	141.0
Number of shares used in per share computations	141.4	141.0
Net income per share - basic	$ 0.20us-gaap_EarningsPerShareBasic	$ 0.18us-gaap_EarningsPerShareBasic
Diluted net income per share:
Net income	$ 28.4us-gaap_NetIncomeLoss	$ 25.4us-gaap_NetIncomeLoss
Weighted average shares outstanding:
Common stock	141.4	141.0
Stock options, awards and employee share purchase plans	2.8	2.8
Number of shares used in per share computations	144.2	143.8
Net income per share - diluted	$ 0.20us-gaap_EarningsPerShareDiluted	$ 0.18us-gaap_EarningsPerShareDiluted
Number of potential common shares excluded from the computation of dilutive net income per share because the effect would have been anti-dilutive	2.5	6.9

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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