Net Income Per Share (Basic and Diluted Income Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income	$ 34.4	$ 108.8
Denominator:
Weighted-average shares used to compute basic net income per share (in shares)	355.3	380.9
Dilutive effect of employee stock awards (in shares)	5.3	7.1
Weighted-average shares used to compute diluted net income per share (in shares)	360.6	388.0
Net income per share
Basic (in dollars per share)	$ 0.10	$ 0.29
Diluted, (in dollars per share)	$ 0.10	$ 0.28
Anti-dilutive shares	10.4	1.6