Balance Sheet Components (Prepaid Expenses and Other Current Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Property, Plant and Equipment [Line Items]
Prepaid expenses	$ 153.7	$ 130.6
Restricted cash	15.5	1.2	$ 53.9
Fair value of financial instruments	58.2	33.8
Equity securities	36.7	32.5
Other current assets	283.8	295.3
Prepaid expenses and other current assets	574.7	518.4
Available-for-sale fixed income investments
Property, Plant and Equipment [Line Items]
Available-for-sale securities	$ 26.8	$ 25.0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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