Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ (148.1)	$ 165.7	$ 179.8	$ 108.8	$ 188.9	$ 172.4	$ 140.0	$ 91.4	$ 306.2	$ 592.7	$ 633.7
Denominator:
Weighted-average shares used to compute basic net income per share									377.7	381.7	390.6
Dilutive effect of employee stock awards									6.5	6.1	8.8
Weighted-average shares used to compute diluted net income per share									384.2	387.8	399.4
Net income per share:
Basic (in dollars per share)	$ (0.40)	$ 0.44	$ 0.47	$ 0.29	$ 0.50	$ 0.45	$ 0.37	$ 0.24	$ 0.81	$ 1.55	$ 1.62
Diluted (in dollars per share)	$ (0.40)	$ 0.43	$ 0.47	$ 0.28	$ 0.49	$ 0.45	$ 0.36	$ 0.23	$ 0.80	$ 1.53	$ 1.59
Anti-dilutive shares excluded from computation of diluted earnings per share									1.1	2.5	3.4