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INVESTMENTS (Tables) | 3 Months Ended
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Mar. 31, 2019
INVESTMENTS |
Summary of investments in available-for-sale |
|  |  |  |  |  |  |  |  |  |  |  |  |  | Continuous |  | Continuous
|  |  |  |  | Gross |  | Gross |  |  |  |  | Unrealized |  | Unrealized
|  |  |  |  | Unrealized |  | Unrealized |  |  |  |  | Loss Position |  | Loss Position
|  | Amortized |  | Holding |  | Holding |  | Fair |  | less than 12 |  | greater than 12
March 31, 2019 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | Months
Available-for-sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 38,964 |  | $ | — |  | $ | — |  | $ | 38,964 |  | $ | — |  | $ | —
Certificates of deposit |  |  | 5,024 |  |  | — |  |  | — |  |  | 5,024 |  |  | — |  |  | —
Municipal securities |  |  | 103,664 |  |  | 30 |  |  | 4 |  |  | 103,690 |  |  | 4 |  |  | —
U.S. government agency securities |  |  | 19,940 |  |  | 7 |  |  | 2 |  |  | 19,945 |  |  | 2 |  |  | —
U.S. treasuries |  |  | 89,267 |  |  | 13 |  |  | 12 |  |  | 89,268 |  |  | 12 |  |  | —
Variable rate demand notes |  |  | 6,806 |  |  | — |  |  | — |  |  | 6,806 |  |  | — |  |  | —
Total |  | $ | 263,665 |  | $ | 50 |  | $ | 18 |  | $ | 263,697 |  | $ | 18 |  | $ | —
|  |  |  |  |  |  |  |  |  |  |  |  |  | Continuous |  | Continuous
|  |  |  |  | Gross |  | Gross |  |  |  |  | Unrealized |  | Unrealized
|  |  |  |  | Unrealized |  | Unrealized |  |  |  |  | Loss Position |  | Loss Position
|  | Amortized |  | Holding |  | Holding |  | Fair |  | less than 12 |  | greater than 12
December 31, 2018 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | Months
Available-for-sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 52,838 |  | $ | — |  | $ | — |  | $ | 52,838 |  | $ | — |  | $ | —
Certificates of deposit |  |  | 14,075 |  |  | — |  |  | — |  |  | 14,075 |  |  | — |  |  | —
Municipal securities |  |  | 151,690 |  |  | 16 |  |  | 62 |  |  | 151,644 |  |  | 62 |  |  | —
U.S. government agency securities |  |  | 19,943 |  |  | — |  |  | 12 |  |  | 19,931 |  |  | 12 |  |  | —
U.S. treasuries |  |  | 78,189 |  |  | — |  |  | 32 |  |  | 78,157 |  |  | 32 |  |  | —
Variable rate demand notes |  |  | 4,005 |  |  | — |  |  | — |  |  | 4,005 |  |  | — |  |  | —
Total |  | $ | 320,740 |  | $ | 16 |  | $ | 106 |  | $ | 320,650 |  | $ | 106 |  | $ | —
Summary of the underlying contractual maturities of investments |
|  | March 31, 2019 |  | December 31, 2018
|  | Amortized Cost |  | Fair Value |  | Amortized Cost |  |  | Fair Value
Less than 1 year: |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 38,964 |  | $ | 38,964 |  | $ | 52,838 |  | $ | 52,838
Municipal securities |  |  | 103,664 |  |  | 103,690 |  |  | 151,690 |  |  | 151,644
U.S. government agency securities |  |  | 19,940 |  |  | 19,945 |  |  | 19,943 |  |  | 19,931
Certificates of deposit |  |  | 5,024 |  |  | 5,024 |  |  | 14,075 |  |  | 14,075
U.S. treasuries |  |  | 89,267 |  |  | 89,268 |  |  | 78,189 |  |  | 78,157
Due 21 - 30 years: |  |  |  |  |  |  |  |  |  |  |  |
Variable rate demand notes |  |  | 5,805 |  |  | 5,805 |  |  | 4,005 |  |  | 4,005
Due 31 - 40 years: |  |  |  |  |  |  |  |  |  |  |  |
Variable rate demand notes |  |  | 1,001 |  |  | 1,001 |  |  | — |  |  | —
Total |  | $ | 263,665 |  | $ | 263,697 |  | $ | 320,740 |  | $ | 320,650

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