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Marketable Securities (Tables) | 6 Months Ended
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Jul. 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 July 31, 2018, January 31, 2018 and July 31, 2017 were as follows:
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Fair |
|  | Cost |  |  | Gains |  |  | (Losses) |  |  | Value |
As of July 31, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 150,648 |  |  | $ | — |  |  | $ | (261 | ) |  | $ | 150,387 |
Municipal and pre-refunded municipal bonds |  |  | 46,904 |  |  |  | 10 |  |  |  | (43 | ) |  |  | 46,871 |
Certificates of deposit |  |  | 908 |  |  |  | — |  |  |  | — |  |  |  | 908 |
|  |  | 198,460 |  |  |  | 10 |  |  |  | (304 | ) |  |  | 198,166 |
Long-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 34,333 |  |  |  | — |  |  |  | (213 | ) |  |  | 34,120 |
Municipal and pre-refunded municipal bonds |  |  | 2,130 |  |  |  | 1 |  |  |  | (7 | ) |  |  | 2,124 |
Mutual funds, held in rabbi trust |  |  | 6,401 |  |  |  | 171 |  |  |  | (1 | ) |  |  | 6,571 |
Certificates of deposit |  |  | 2,699 |  |  |  | — |  |  |  | — |  |  |  | 2,699 |
|  |  | 45,563 |  |  |  | 172 |  |  |  | (221 | ) |  |  | 45,514 |
|  | $ | 244,023 |  |  | $ | 182 |  |  | $ | (525 | ) |  | $ | 243,680 |
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 July 31, 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 72,805 |  |  | $ | 4 |  |  | $ | (61 | ) |  | $ | 72,748 |
Municipal and pre-refunded municipal bonds |  |  | 36,630 |  |  |  | 15 |  |  |  | (13 | ) |  |  | 36,632 |
Certificates of deposit |  |  | 815 |  |  |  | — |  |  |  | — |  |  |  | 815 |
|  |  | 110,250 |  |  |  | 19 |  |  |  | (74 | ) |  |  | 110,195 |
Long-term Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 14,906 |  |  |  | — |  |  |  | (22 | ) |  |  | 14,884 |
Municipal and pre-refunded municipal bonds |  |  | 3,057 |  |  |  | 9 |  |  |  | (2 | ) |  |  | 3,064 |
Mutual funds, held in rabbi trust |  |  | 5,183 |  |  |  | 103 |  |  |  | — |  |  |  | 5,286 |
Certificates of deposit |  |  | 2,726 |  |  |  | — |  |  |  | — |  |  |  | 2,726 |
|  |  | 25,872 |  |  |  | 112 |  |  |  | (24 | ) |  |  | 25,960 |
|  | $ | 136,122 |  |  | $ | 131 |  |  | $ | (98 | ) |  | $ | 136,155 |

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