Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 168.4	$ 99.3	$ 46.2	$ 31.1	$ 192.2	$ 223.8	$ 116.5	$ 34.4	$ 345.0	$ 566.9	$ 306.2
Denominator:
Weighted-average shares used to compute basic net income per share									343.2	349.0	377.7
Dilutive effect of employee stock awards									5.0	5.4	6.5
Weighted-average shares used to compute diluted net income per share									348.2	354.4	384.2
Net income per share:
Basic (in dollars per share)	$ 0.50	$ 0.29	$ 0.13	$ 0.09	$ 0.56	$ 0.65	$ 0.33	$ 0.10	$ 1.01	$ 1.62	$ 0.81
Diluted (in dollars per share)	$ 0.49	$ 0.29	$ 0.13	$ 0.09	$ 0.55	$ 0.64	$ 0.33	$ 0.10	$ 0.99	$ 1.60	$ 0.80
Anti-dilutive shares excluded from computation of diluted earnings per share									4.7	3.9	1.1