Balance Sheet Components (Prepaid Expenses and Other Current Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Property, Plant and Equipment [Line Items]
Prepaid expenses	$ 259.9	$ 267.8
Restricted cash	6.2	5.6	$ 15.5
Fair value of financial instruments	103.6	118.6
Equity securities	47.5	45.8
Other current assets	1,591.5	1,236.1
Prepaid expenses and other current assets	2,040.4	1,707.4
Available-for-sale fixed income investments
Property, Plant and Equipment [Line Items]
Available-for-sale securities	$ 37.9	$ 39.1