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Balance Sheet Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2019
Debt securities, available for sale: |  |
Amortized Cost | $ 210.7 | $ 1,033.9
Gross Unrealized Gains | 0.0 | 0.5
Gross Unrealized Losses | (0.1) | (0.1)
Estimated Market Value | 210.6 | 1,034.3
Total marketable securities |  |
Cost or Amortized Cost | 212.7 |
Gross Unrealized Gains | 36.0 |
Gross Unrealized Losses | (0.1) |
Estimated Market Value | 248.6 | 1,034.3
U.S. government agencies |  |
Debt securities, available for sale: |  |
Amortized Cost | 173.2 | 698.6
Gross Unrealized Gains | 0.0 | 0.4
Gross Unrealized Losses | (0.1) | 0.0
Estimated Market Value | 173.1 | 699.0
Commercial paper |  |
Debt securities, available for sale: |  |
Amortized Cost | 36.2 | 277.9
Gross Unrealized Gains | 0.0 | 0.1
Gross Unrealized Losses | 0.0 | (0.1)
Estimated Market Value | 36.2 | 277.9
Corporate debt |  |
Debt securities, available for sale: |  |
Amortized Cost | 1.3 | 57.4
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated Market Value | 1.3 | $ 57.4
Equity investment in Tandem Diabetes Care, Inc. |  |
Equity investment in Tandem Diabetes Care, Inc. |  |
Cost or Amortized Cost | 2.0 |
Gross Unrealized Gains | 36.0 |
Gross Unrealized Losses | 0.0 |
Estimated Market Value | $ 38.0 |

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