Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 99.3	$ 223.8	$ 176.6	$ 374.7
Denominator:
Weighted-average shares used to compute basic net income per share (in shares)	342.2	346.2	345.5	350.1
Dilutive effect of employee stock awards (in shares)	3.3	4.3	4.6	5.1
Weighted-average shares used to compute diluted net income per share (in shares)	345.5	350.5	350.1	355.2
Net income per share
Basic (in dollars per share)	$ 0.29	$ 0.65	$ 0.51	$ 1.07
Diluted, (in dollars per share)	$ 0.29	$ 0.64	$ 0.50	$ 1.05
Anti-dilutive shares (in shares)	2.8	4.2	2.7	5.0