Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 223.8	$ 165.7	$ 374.7	$ 454.3
Denominator:
Weighted-average shares used to compute basic net income per share (in shares)	346.2	378.3	350.1	380.0
Dilutive effect of employee stock awards (in shares)	4.3	4.4	5.1	6.5
Weighted-average shares used to compute diluted net income per share (in shares)	350.5	382.7	355.2	386.5
Net income per share
Basic (in dollars per share)	$ 0.65	$ 0.44	$ 1.07	$ 1.20
Diluted, (in dollars per share)	$ 0.64	$ 0.43	$ 1.05	$ 1.18
Anti-dilutive shares (in shares)	4.2	1.0	5.0	1.2