Balance Sheet Components (Prepaid Expenses and Other Current Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Property, Plant and Equipment [Line Items]
Prepaid expenses	$ 168.2	$ 156.7
Restricted cash	1.1	15.5	$ 1.2
Fair value of financial instruments	19.1	43.3
Equity securities	39.0	39.0
Other current assets	336.1	270.7
Prepaid expenses and other current assets	602.1	552.0
Available-for-sale fixed income investments
Property, Plant and Equipment [Line Items]
Available-for-sale securities	$ 38.6	$ 26.8