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Investments (Tables) | 12 Months Ended
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Apr. 30, 2020
Investments |
Schedule of investments | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | April 30,
​ |  | 2020 |  | 2019
​ | ​ | (In thousands)
Short-term investments: | ​ | ​ | ​ | ​ | ​ | ​
Held-to-maturity securities: | ​ | ​ | ​ | ​ | ​ | ​
Municipal securities | ​ | $ | — | ​ | $ | 5,332
U.S. government securities | ​ | ​ | — | ​ | ​ | 63,205
Corporate bonds | ​ |  | — | ​ |  | 81,950
Total held-to-maturity investments | ​ | ​ | — | ​ | ​ | 150,487
Available-for-sale securities: | ​ | ​ | ​ | ​ | ​ | ​
Municipal securities | ​ | ​ | 5,244 | ​ | ​ | —
U.S. government securities | ​ | ​ | 33,771 | ​ | ​ | —
Corporate bonds | ​ | ​ | 8,492 | ​ | ​ | —
Total available-for-sale investments | ​ | ​ | 47,507 | ​ | ​ | —
Total short-term investments | ​ | $ | 47,507 | ​ | $ | 150,487
Long-term investments: | ​ | ​ | ​ | ​ | ​ | ​
Held-to-maturity securities: | ​ | ​ | ​ | ​ | ​ | ​
U.S. government securities | ​ | $ | — | ​ | $ | 7,404
Corporate bonds | ​ |  | — | ​ |  | 1,982
Total held-to-maturity investments | ​ |  | — | ​ |  | 9,386
Available-for-sale securities: | ​ | ​ | ​ | ​ | ​ | ​
Municipal securities | ​ | ​ | 1,592 | ​ | ​ | —
U.S. government securities | ​ | ​ | 8,996 | ​ | ​ | —
Total available-for-sale investments | ​ |  | 10,588 | ​ |  | —
Equity method investments | ​ | ​ | ​ | ​ | ​ | ​
Investment in limited partnership fund | ​ |  | 4,442 | ​ |  | —
Total equity method investments | ​ |  | 4,442 | ​ |  | —
Total long-term investments | ​ | $ | 15,030 | ​ | $ | 9,386
Schedule of activity related to available-for-sale investments recorded in short-term and long-term investments | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | April 30, 2020
​ |  | ​ | ​ |  | Gross |  | Gross |  | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
​ | ​ | Cost | ​ | Gains | ​ | Losses | ​ | Value
Municipal securities | ​ | $ | 6,807 | ​ | $ | 29 | ​ | $ | — | ​ | $ | 6,836
U.S. government securities | ​ | ​ | 42,730 | ​ | ​ | 41 | ​ | ​ | (4) | ​ | ​ | 42,767
Corporate bonds | ​ | ​ | 8,495 | ​ | ​ | — | ​ | ​ | (3) | ​ | ​ | 8,492
Total held-to-maturity investments | ​ | $ | 58,032 | ​ | $ | 70 | ​ | $ | (7) | ​ | $ | 58,095
Schedule of amortized cost, gross unrealized gains, gross unrealized losses, and estimated fair value of available-for-sale investments | April 30, 2020, are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Cost |  | Fair Value |
Due within one year | ​ | $ | 47,457 | ​ | $ | 47,507 | ​
Due after one year through five years | ​ |  | 10,575 | ​ |  | 10,588 | ​
Total | ​ | $ | 58,032 | ​ | $ | 58,095 | ​
Held to maturity securities |
Investments |
Schedule of amortized cost, gross unrealized gains, gross unrealized losses, and estimated fair value of held-to-maturity investments | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ | ​ |  | Net
​ | ​ | Carrying | ​ | Unrealized
​ | ​ | Amount | ​ | Gains
Municipal securities | ​ | $ | 23,706 | ​ | $ | 34
U.S. government securities | ​ | ​ | 41,465 | ​ | ​ | 38
Corporate bonds | ​ | ​ | 85,679 | ​ | ​ | 73
Certificate of deposits | ​ | ​ | 1,017 | ​ | ​ | 5
Total held-to-maturity investments | ​ | $ | 151,867 | ​ | $ | 150
​ The Company held no held-to maturity securities at April 30, 2020. The amortized cost, gross unrealized losses, and estimated fair value of the held-to-maturity securities as of April 30, 2019 are as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair |
​ | ​ | Cost |  | Gains | ​ | Losses |  | Value |
Municipal securities |  | $ | 5,332 | ​ | $ | 2 | ​ | $ | (1) | ​ | $ | 5,333 | ​
U.S. government securities | ​ |  | 70,609 | ​ | ​ | 78 | ​ | ​ | (52) | ​ | ​ | 70,635 | ​
Corporate bonds | ​ |  | 83,932 | ​ | ​ | 20 | ​ | ​ | (5) | ​ | ​ | 83,947 | ​
Total held-to-maturity investments |  | $ | 159,873 |  | $ | 100 | ​ | $ | (58) |  | $ | 159,915 | ​
Schedule of amortized cost and fair value by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair |
​ | ​ | Cost |  | Gains | ​ | Losses |  | Value |
Municipal securities |  | $ | 5,332 | ​ | $ | 2 | ​ | $ | (1) | ​ | $ | 5,333 | ​
U.S. government securities | ​ |  | 70,609 | ​ | ​ | 78 | ​ | ​ | (52) | ​ | ​ | 70,635 | ​
Corporate bonds | ​ |  | 83,932 | ​ | ​ | 20 | ​ | ​ | (5) | ​ | ​ | 83,947 | ​
Total held-to-maturity investments |  | $ | 159,873 |  | $ | 100 | ​ | $ | (58) |  | $ | 159,915 | ​

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