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Financial Instruments - Summary of Contractual Maturities (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Estimated Fair Value    
Due in one year or less $ 1,278.9 $ 1,562.2
Due after one year through five years 722.6 1,234.5
Due after five years 49.5 173.6
Total marketable debt securities 2,051.0 2,970.3
Amortized Cost    
Due in one year or less 1,278.6 1,560.8
Due after one year through five years 721.3 1,230.4
Due after five years 49.2 173.8
Amortized Cost $ 2,049.1 $ 2,965.0