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Balance Sheet Details - Short-term Marketable Securities (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Debt securities, available for sale: |  |
Amortized Cost | $ 1,898.6 | $ 1,890.0
Gross Unrealized Gains | 0.2 | 0.2
Gross Unrealized Losses | 0.0 | (0.1)
Estimated Market Value | 1,898.8 | 1,890.1
U.S. government agencies |  |
Debt securities, available for sale: |  |
Amortized Cost | 1,502.0 | 1,570.4
Gross Unrealized Gains | 0.2 | 0.1
Gross Unrealized Losses | 0.0 | (0.1)
Estimated Market Value | 1,502.2 | 1,570.4
Commercial paper |  |
Debt securities, available for sale: |  |
Amortized Cost | 313.8 | 258.7
Gross Unrealized Gains | 0.0 | 0.1
Gross Unrealized Losses | 0.0 | 0.0
Estimated Market Value | 313.8 | 258.8
Corporate debt |  |
Debt securities, available for sale: |  |
Amortized Cost | 82.8 | 60.9
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses |  | 0.0
Estimated Market Value | $ 82.8 | $ 60.9

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