Balance Sheet Components (Prepaid Expenses and Other Current Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Property, Plant and Equipment [Line Items]
Prepaid expenses	$ 189.1	$ 156.7
Restricted cash	2.6	15.5	$ 15.5
Fair value of financial instruments	51.1	43.3
Equity securities	40.9	39.0
Other current assets	361.3	270.7
Prepaid expenses and other current assets	684.1	552.0
Available-for-sale fixed income investments
Property, Plant and Equipment [Line Items]
Available-for-sale securities	$ 39.1	$ 26.8