Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income	$ 30.8	$ 145.4	$ 61.2	$ 20.4	$ 168.4	$ 99.3	$ 46.2	$ 31.1	$ 257.8	$ 345.0	$ 566.9
Denominator:
Weighted-average shares used to compute basic net income per share (in shares)									330.4	343.2	349.0
Dilutive effect of employee stock awards (in shares)									4.8	5.0	5.4
Weighted-average shares used to compute diluted net income per share (in shares)									335.2	348.2	354.4
Net income per share:
Basic (in dollars per share)	$ 0.09	$ 0.44	$ 0.18	$ 0.06	$ 0.50	$ 0.29	$ 0.13	$ 0.09	$ 0.78	$ 1.01	$ 1.62
Diluted (in dollars per share)	$ 0.09	$ 0.43	$ 0.18	$ 0.06	$ 0.49	$ 0.29	$ 0.13	$ 0.09	$ 0.77	$ 0.99	$ 1.60
Anti-dilutive shares (in shares)									5.3	4.7	3.9