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Marketable Securities (Tables) | 9 Months Ended
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Oct. 31, 2018
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, 2018, January 31, 2018 and October 31, 2017 were as follows:
|  | 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 |
As of January 31, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 111,612 |  |  | $ | — |  |  | $ | (184 | ) |  | $ | 111,428 |
Municipal and pre-refunded municipal bonds |  |  | 52,474 |  |  |  | 11 |  |  |  | (39 | ) |  |  | 52,446 |
Certificates of deposit |  |  | 1,251 |  |  |  | — |  |  |  | — |  |  |  | 1,251 |
|  |  | 165,337 |  |  |  | 11 |  |  |  | (223 | ) |  |  | 165,125 |
Long-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 39,853 |  |  |  | — |  |  |  | (228 | ) |  |  | 39,625 |
Municipal and pre-refunded municipal bonds |  |  | 9,873 |  |  |  | 8 |  |  |  | (24 | ) |  |  | 9,857 |
Mutual funds, held in rabbi trust |  |  | 5,973 |  |  |  | 274 |  |  |  | (10 | ) |  |  | 6,237 |
Certificates of deposit |  |  | 2,969 |  |  |  | — |  |  |  | — |  |  |  | 2,969 |
|  |  | 58,668 |  |  |  | 282 |  |  |  | (262 | ) |  |  | 58,688 |
|  | $ | 224,005 |  |  | $ | 293 |  |  | $ | (485 | ) |  | $ | 223,813 |
As of October 31, 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 67,275 |  |  | $ | 2 |  |  | $ | (71 | ) |  | $ | 67,206 |
Municipal and pre-refunded municipal bonds |  |  | 24,676 |  |  |  | 4 |  |  |  | (14 | ) |  |  | 24,666 |
Certificates of deposit |  |  | 1,356 |  |  |  | — |  |  |  | — |  |  |  | 1,356 |
|  |  | 93,307 |  |  |  | 6 |  |  |  | (85 | ) |  |  | 93,228 |
Long-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 30,051 |  |  |  | 2 |  |  |  | (87 | ) |  |  | 29,966 |
Municipal and pre-refunded municipal bonds |  |  | 1,362 |  |  |  | — |  |  |  | (2 | ) |  |  | 1,360 |
Mutual funds, held in rabbi trust |  |  | 5,639 |  |  |  | 109 |  |  |  | (2 | ) |  |  | 5,746 |
Certificates of deposit |  |  | 4,182 |  |  |  | — |  |  |  | — |  |  |  | 4,182 |
|  |  | 41,234 |  |  |  | 111 |  |  |  | (91 | ) |  |  | 41,254 |
|  | $ | 134,541 |  |  | $ | 117 |  |  | $ | (176 | ) |  | $ | 134,482 |

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