Cash Equivalents and Investments - Maturities of Fixed Income Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Amortized Cost	$ 928.4	$ 1,407.3
Estimated Fair Value
Total	969.9	1,451.5
Fixed Income Securities
Amortized Cost
Due in less than one year	430.7
Due between one and five years	478.5
Amortized Cost	909.2	1,389.0
Estimated Fair Value
Due in less than one year	431.9
Due between one and five years	481.4
Total	$ 913.3	$ 1,395.8

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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