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Balance Sheet Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Debt securities, available for sale: |  |  |
Amortized Cost | $ 72.9 |  | $ 210.7
Gross Unrealized Gains | 0.0 |  | 0.0
Gross Unrealized Losses | 0.0 |  | (0.1)
Estimated Market Value | 72.9 | $ 210.6 | 210.6
Total marketable securities |  |  |
Cost or Amortized Cost |  |  | 212.7
Gross Unrealized Gains |  |  | 36.0
Gross Unrealized Losses |  |  | (0.1)
Estimated Market Value | 72.9 |  | 248.6
Equity Securities Net Gain |  |  |
Net gains recognized during the period on equity securities | (4.2) | 7.4 |
U.S. government agencies |  |  |
Debt securities, available for sale: |  |  |
Amortized Cost | 54.8 |  | 173.2
Gross Unrealized Gains | 0.0 |  | 0.0
Gross Unrealized Losses | 0.0 |  | (0.1)
Estimated Market Value | 54.8 | 173.1 | 173.1
Commercial paper |  |  |
Debt securities, available for sale: |  |  |
Amortized Cost | 3.0 |  | 36.2
Gross Unrealized Gains | 0.0 |  | 0.0
Gross Unrealized Losses | 0.0 |  | 0.0
Estimated Market Value | 3.0 | 36.2 | 36.2
Corporate debt |  |  |
Debt securities, available for sale: |  |  |
Amortized Cost | 15.1 |  | 1.3
Gross Unrealized Gains | 0.0 |  | 0.0
Gross Unrealized Losses | 0.0 |  | 0.0
Estimated Market Value | $ 15.1 | $ 1.3 | 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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