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Balance Sheet Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Debt securities, available for sale: |  |  |
Debt securities, available-for-sale, amortized cost | $ 210.7 | $ 107.3 |
Debt Securities, gross unrealized gains | 0.0 | 0.0 |
Debt securities, gross unrealized losses | (0.1) | (0.2) |
Short-term marketable securities | 210.6 | 107.1 |
Debt Securities, Available-for-sale and Equity Securities, Amortized Cost | 212.7 |  |
Total marketable securities, cost or amortized cost | 248.6 | 107.1 |
Total marketable securities, gross unrealized gains | 36.0 |  |
Total marketable securities, gross unrealized losses | (0.1) |  |
Debt securities with maturities of less than 12 months | 92.7 |  |
Debt securities with maturities of more than 12 months | $ 14.4 |  |
Debt securities held maximum contractual maturities | 16 months |  |
Net gain recognized on equity securities |  |  |
Net gains recognized during the period on equity securities | $ 80.1 | 0.0 | $ 0.0
Less: Net gains recognized during the period on equity securities sold during the period | (44.1) | 0.0 | 0.0
Unrealized gains recognized during the reporting period on equity securities still held at the reporting date | 36.0 | 0.0 | $ 0.0
U.S. government agencies |  |  |
Debt securities, available for sale: |  |  |
Debt securities, available-for-sale, amortized cost | 173.2 | 87.5 |
Debt Securities, gross unrealized gains | 0.0 | 0.0 |
Debt securities, gross unrealized losses | (0.1) | (0.2) |
Short-term marketable securities | 173.1 | 87.3 |
Commercial paper |  |  |
Debt securities, available for sale: |  |  |
Debt securities, available-for-sale, amortized cost | 36.2 | 14.7 |
Debt Securities, gross unrealized gains | 0.0 | 0.0 |
Debt securities, gross unrealized losses | 0.0 | 0.0 |
Short-term marketable securities | 36.2 | 14.7 |
Corporate debt |  |  |
Debt securities, available for sale: |  |  |
Debt securities, available-for-sale, amortized cost | 1.3 | 5.1 |
Debt Securities, gross unrealized gains | 0.0 | 0.0 |
Debt securities, gross unrealized losses | 0.0 | 0.0 |
Short-term marketable securities | 1.3 | $ 5.1 |
Equity investment in Tandem Diabetes Care, Inc |  |  |
Short-Term Marketable Securities |  |  |
Equity investment in Tandem Diabetes Care, Inc., cost or amortized cost | 2.0 |  |
Equity investment in Tendem Diabetes Care, Inc., gross unrealized gains | 36.0 |  |
Equity investment in Tandem Diabetes Care, inc., gross unrealized losses | 0.0 |  |
Equity Invesment in Tandem Diabetes Care, Inc., estimated market value | $ 38.0 |  |

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