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Cash, Cash Equivalents and Short-Term Investments - Contractual Maturities of Short Term Investments (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Amortized cost  
Due within one year $ 327.6
Due between one to three years 242.9
Total 570.5
Estimated fair value  
Due within one year 327.5
Due between one to three years 242.5
Total $ 570.0