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INVESTMENTS (Tables) | 12 Months Ended
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Dec. 31, 2018
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
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 |  | $ | —
|  |  |  |  |  |  |  |  |  |  |  |  |  | Continuous |  | Continuous
|  |  |  |  | Gross |  | Gross |  |  |  |  | Unrealized |  | Unrealized
|  |  |  |  | Unrealized |  | Unrealized |  |  |  |  | Loss Position |  | Loss Position
|  | Amortized |  | Holding |  | Holding |  | Fair |  | less than 12 |  | greater than 12
December 31, 2017 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | Months
Available-for-sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 81,026 |  | $ | — |  | $ | — |  | $ | 81,026 |  | $ | — |  | $ | —
Certificates of deposit |  |  | 11,869 |  |  | — |  |  | — |  |  | 11,869 |  |  | — |  |  | —
Municipal securities |  |  | 469,604 |  |  | 1 |  |  | 740 |  |  | 468,865 |  |  | 740 |  |  | —
U.S. government agency securities |  |  | 61,307 |  |  | — |  |  | 88 |  |  | 61,219 |  |  | 88 |  |  | —
Variable rate demand notes |  |  | 49,954 |  |  | — |  |  | — |  |  | 49,954 |  |  | — |  |  | —
Long-term: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. government agency securities |  |  | 2,369 |  |  | — |  |  | 3 |  |  | 2,366 |  |  | 3 |  |  | —
Total |  | $ | 676,129 |  | $ | 1 |  | $ | 831 |  | $ | 675,299 |  | $ | 831 |  | $ | —
Summary of the underlying contractual maturities of investments |
|  | December 31, 2018 |  | December 31, 2017
|  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Less than 1 year: |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 52,838 |  | $ | 52,838 |  | $ | 81,026 |  | $ | 81,026
Municipal securities |  |  | 151,690 |  |  | 151,644 |  |  | 469,604 |  |  | 468,865
U.S. government agency securities |  |  | 19,943 |  |  | 19,931 |  |  | 61,307 |  |  | 61,219
Certificates of deposit |  |  | 14,075 |  |  | 14,075 |  |  | 11,869 |  |  | 11,869
U.S. treasuries |  |  | 78,189 |  |  | 78,157 |  |  | — |  |  | —
Due 1 - 10 years: |  |  |  |  |  |  |  |  |  |  |  |
U.S. government agency securities |  |  | — |  |  | — |  |  | 2,369 |  |  | 2,366
Variable rate demand notes |  |  | — |  |  | — |  |  | 6,366 |  |  | 6,366
Due 11 - 20 years: |  |  |  |  |  |  |  |  |  |  |  |
Variable rate demand notes |  |  | — |  |  | — |  |  | 28,377 |  |  | 28,377
Due 21 - 30 years: |  |  |  |  |  |  |  |  |  |  |  |
Variable rate demand notes |  |  | 4,005 |  |  | 4,005 |  |  | 15,211 |  |  | 15,211
Total |  | $ | 320,740 |  | $ | 320,650 |  | $ | 676,129 |  | $ | 675,299

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