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Balance Sheet Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Debt securities, available for sale: |  |  |  |  |
Amortized Cost | $ 667.8 |  | $ 667.8 |  | $ 210.7
Gross Unrealized Gains | 0.5 |  | 0.5 |  | 0.0
Gross Unrealized Losses | 0.0 |  | 0.0 |  | (0.1)
Estimated Market Value | 668.3 |  | 668.3 |  | 210.6
Total marketable securities |  |  |  |  |
Cost or Amortized Cost |  |  |  |  | 212.7
Gross Unrealized Gains |  |  |  |  | 36.0
Gross Unrealized Losses |  |  |  |  | (0.1)
Estimated Market Value | 668.3 |  | 668.3 |  | 248.6
Equity Securities Net Gain |  |  |  |  |
Net gains recognized during the period on equity securities | 0.0 | $ 42.7 | (4.2) | $ 50.1 |
U.S. government agencies |  |  |  |  |
Debt securities, available for sale: |  |  |  |  |
Amortized Cost | 531.2 |  | 531.2 |  | 173.2
Gross Unrealized Gains | 0.4 |  | 0.4 |  | 0.0
Gross Unrealized Losses | 0.0 |  | 0.0 |  | (0.1)
Estimated Market Value | 531.6 |  | 531.6 |  | 173.1
Commercial paper |  |  |  |  |
Debt securities, available for sale: |  |  |  |  |
Amortized Cost | 98.8 |  | 98.8 |  | 36.2
Gross Unrealized Gains | 0.1 |  | 0.1 |  | 0.0
Gross Unrealized Losses | 0.0 |  | 0.0 |  | 0.0
Estimated Market Value | 98.9 |  | 98.9 |  | 36.2
Corporate debt |  |  |  |  |
Debt securities, available for sale: |  |  |  |  |
Amortized Cost | 37.8 |  | 37.8 |  | 1.3
Gross Unrealized Gains | 0.0 |  | 0.0 |  | 0.0
Gross Unrealized Losses | 0.0 |  | 0.0 |  | 0.0
Estimated Market Value | $ 37.8 |  | $ 37.8 |  | 1.3
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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