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Marketable Securities (Tables) | 9 Months Ended
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Oct. 31, 2019
Investments Debt And Equity Securities [Abstract] |
Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Available-For-Sale Securities | The amortized cost, gross unrealized gains (losses) and fair value of available-for-sale securities by major security type and class of security as of October 31, 2019, January 31, 2019 and October 31, 2018 were as follows:
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Fair |
|  | Cost |  |  | Gains |  |  | (Losses) |  |  | Value |
As of October 31, 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 125,040 |  |  | $ | 310 |  |  | $ | (12 | ) |  | $ | 125,338 |
Municipal and pre-refunded municipal bonds |  |  | 39,256 |  |  |  | 38 |  |  |  | (14 | ) |  |  | 39,280 |
Federal government agencies |  |  | 1,622 |  |  |  | 4 |  |  |  | — |  |  |  | 1,626 |
Certificates of deposit |  |  | 2,466 |  |  |  | — |  |  |  | — |  |  |  | 2,466 |
Commercial paper |  |  | 1,987 |  |  |  | — |  |  |  | — |  |  |  | 1,987 |
|  |  | 170,371 |  |  |  | 352 |  |  |  | (26 | ) |  |  | 170,697 |
Long-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 45,556 |  |  |  | 270 |  |  |  | (26 | ) |  |  | 45,800 |
Municipal and pre-refunded municipal bonds |  |  | 20,795 |  |  |  | 13 |  |  |  | (35 | ) |  |  | 20,773 |
Federal government agencies |  |  | 6,365 |  |  |  | — |  |  |  | (15 | ) |  |  | 6,350 |
Mutual funds, held in rabbi trust |  |  | 7,661 |  |  |  | 144 |  |  |  | (1 | ) |  |  | 7,804 |
Certificates of deposit |  |  | 2,394 |  |  |  | — |  |  |  | — |  |  |  | 2,394 |
|  |  | 82,771 |  |  |  | 427 |  |  |  | (77 | ) |  |  | 83,121 |
|  | $ | 253,142 |  |  | $ | 779 |  |  | $ | (103 | ) |  | $ | 253,818 |
As of January 31, 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 227,287 |  |  | $ | 24 |  |  | $ | (214 | ) |  | $ | 227,097 |
Municipal and pre-refunded municipal bonds |  |  | 43,677 |  |  |  | 15 |  |  |  | (18 | ) |  |  | 43,674 |
Federal government agencies |  |  | 1,458 |  |  |  | — |  |  |  | — |  |  |  | 1,458 |
Certificates of deposit |  |  | 1,050 |  |  |  | — |  |  |  | — |  |  |  | 1,050 |
Commercial paper |  |  | 2,979 |  |  |  | — |  |  |  | — |  |  |  | 2,979 |
Treasury bills |  |  | 2,975 |  |  |  | — |  |  |  | (1 | ) |  |  | 2,974 |
|  |  | 279,426 |  |  |  | 39 |  |  |  | (233 | ) |  |  | 279,232 |
Long-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 34,265 |  |  |  | 34 |  |  |  | (63 | ) |  |  | 34,236 |
Municipal and pre-refunded municipal bonds |  |  | 7,554 |  |  |  | 7 |  |  |  | (3 | ) |  |  | 7,558 |
Federal government agencies |  |  | 6,603 |  |  |  | 2 |  |  |  | (1 | ) |  |  | 6,604 |
Mutual funds, held in rabbi trust |  |  | 6,301 |  |  |  | 450 |  |  |  | — |  |  |  | 6,751 |
Certificates of deposit |  |  | 2,143 |  |  |  | — |  |  |  | — |  |  |  | 2,143 |
|  |  | 56,866 |  |  |  | 493 |  |  |  | (67 | ) |  |  | 57,292 |
|  | $ | 336,292 |  |  | $ | 532 |  |  | $ | (300 | ) |  | $ | 336,524 |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Fair |
|  | Cost |  |  | Gains |  |  | (Losses) |  |  | Value |
As of October 31, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 200,910 |  |  | $ | — |  |  | $ | (466 | ) |  | $ | 200,444 |
Municipal and pre-refunded municipal bonds |  |  | 33,291 |  |  |  | — |  |  |  | (59 | ) |  |  | 33,232 |
Certificates of deposit |  |  | 755 |  |  |  | — |  |  |  | — |  |  |  | 755 |
Commercial paper |  |  | 2,960 |  |  |  | — |  |  |  | — |  |  |  | 2,960 |
|  |  | 237,916 |  |  |  | — |  |  |  | (525 | ) |  |  | 237,391 |
Long-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 24,793 |  |  |  | — |  |  |  | (175 | ) |  |  | 24,618 |
Municipal and pre-refunded municipal bonds |  |  | 2,573 |  |  |  | — |  |  |  | (7 | ) |  |  | 2,566 |
Mutual funds, held in rabbi trust |  |  | 6,932 |  |  |  | — |  |  |  | (473 | ) |  |  | 6,459 |
Certificates of deposit |  |  | 2,390 |  |  |  | — |  |  |  | — |  |  |  | 2,390 |
|  |  | 36,688 |  |  |  | — |  |  |  | (655 | ) |  |  | 36,033 |
|  | $ | 274,604 |  |  | $ | — |  |  | $ | (1,180 | ) |  | $ | 273,424 |

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