Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Numerator
Net income	$ 91.4	$ 80.2
Denominator
Weighted-average shares used to compute basic net income per share	383.2	407.1
Dilutive effect of employee stock awards	6.1	7.1
Weighted-average shares used to compute diluted net income per share	389.3	414.2
Net income per share:
Basic, in dollars per share	$ 0.24	$ 0.20
Diluted, in dollars per share	$ 0.23	$ 0.19
Net Income per Share Textuals
Potential anti-dilutive shares	4.0	6.4