Cash Equivalents and Investments - Maturities of Fixed Income Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Amortized Cost	$ 1,776.1	$ 2,223.6
Estimated Fair Value
Total	1,811.7	2,256.8
Fixed Income Securities
Amortized Cost
Due in less than one year	1,641.1
Due between one and five years	120.4
Amortized Cost	1,761.5	2,207.0
Estimated Fair Value
Due in less than one year	1,640.0
Due between one and five years	119.7
Total	$ 1,759.7	$ 2,202.8

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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