Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 31.1	$ 34.4
Denominator:
Weighted-average shares used to compute basic net income per share (in shares)	348.1	355.3
Dilutive effect of employee stock awards (in shares)	4.6	5.3
Weighted-average shares used to compute diluted net income per share (in shares)	352.7	360.6
Net income per share
Basic (in dollars per share)	$ 0.09	$ 0.10
Diluted, (in dollars per share)	$ 0.09	$ 0.10
Anti-dilutive shares (in shares)	5.0	10.4