Net Income per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator
Net income	$ 97.9	$ 57.7	$ 188.9	$ 74.0
Denominator
Weighted-average shares used to compute basic net income per share	503.0	527.8	503.8	528.0
Effect of dilutive securities:
Dilutive effect of employee stock awards	3.3	4.0	6.6	5.8
Weighted-average shares used to compute diluted net income per share	506.3	531.8	510.4	533.8
Net income per share:
Basic	$ 0.19	$ 0.11	$ 0.37	$ 0.14
Diluted	$ 0.19	$ 0.11	$ 0.37	$ 0.14
Net Income per Share Textuals
Anti-dilutive shares excluded from computation of diluted earnings per share	39.4	42.3	24.3	33.2