Investments (Details) - USD ($) $ in Thousands	Apr. 30, 2017	Apr. 30, 2016
Short-term investments:
Total held-to-maturity and short-term investments	$ 119,971	$ 103,404
Long-term investments:
Total long-term investments	42,096	33,859
Certificates of deposit
Short-term investments:
Total held-to-maturity and short-term investments	2,500
Held-to-maturity securities
Short-term investments:
Total held-to-maturity and short-term investments	119,971	103,404
Long-term investments:
Total long-term investments	39,599	31,079
Held-to-maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	47,437	42,179
Long-term investments:
Total long-term investments	8,942
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	14,515	21,184
Long-term investments:
Total long-term investments	22,540	7,518
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total held-to-maturity and short-term investments	55,519	40,041
Long-term investments:
Total long-term investments	8,117	23,561
Available-for-sale securities
Long-term investments:
Total long-term investments	2,497	2,780
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments	$ 2,497	$ 2,780