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Cash, Cash Equivalents and Short-Term Investments - Contractual Maturities of Short Term Investments (Details) $ in Millions | Dec. 31, 2018 USD ($)
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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

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