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Investments - Contractual Maturities of Available-for-Sale Marketable Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Amortized Cost    
Due within one year $ 813.4 $ 708.2
Due in one to three years 409.6 441.5
Total 1,223.0 1,149.7
Fair Value    
Due within one year 812.3 705.8
Due in one to three years 410.5 440.4
Total $ 1,222.8 $ 1,146.2