Cash Equivalents and Investments - Maturities of Fixed Income Securities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Amortized Cost	$ 933.3	$ 1,407.3
Estimated Fair Value
Total	976.3	1,451.5
Fixed Income Securities
Amortized Cost
Due in less than one year	420.9
Due between one and five years	491.2
Amortized Cost	912.1	1,389.0
Estimated Fair Value
Due in less than one year	422.0
Due between one and five years	493.2
Total	$ 915.2	$ 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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