Cash Equivalents and Investments - Maturities of Available for Sale Investments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Amortized Cost	$ 1,407.3	$ 1,633.1
Estimated Fair Value
Total	1,451.5	1,671.4
Fixed Income Securities
Amortized Cost
Due in less than one year	738.0
Due between one and five years	651.0
Amortized Cost	1,389.0	1,614.0
Estimated Fair Value
Due in less than one year	739.2
Due between one and five years	656.6
Total	$ 1,395.8	$ 1,614.9

Amortized cost 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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