Investments (Details) - USD ($) $ in Thousands	Jan. 27, 2018	Apr. 30, 2017
Short-term investments:
Total held-to-maturity and short-term investments	$ 109,543	$ 119,971
Long-term investments:
Total long-term investments	38,822	42,096
Held-to-maturity securities
Short-term investments:
Total held-to-maturity and short-term investments	109,543	119,971
Long-term investments:
Total long-term investments	36,704	39,599
Held-to-maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	53,728	47,437
Long-term investments:
Total long-term investments	1,258	8,942
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	28,607	14,515
Long-term investments:
Total long-term investments	29,467	22,540
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total held-to-maturity and short-term investments	27,208	55,519
Long-term investments:
Total long-term investments	5,979	8,117
Held-to-maturity securities | Certificates of deposit
Short-term investments:
Total held-to-maturity and short-term investments		2,500
Available-for-sale securities
Long-term investments:
Total long-term investments	2,118	2,497
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments	$ 2,118	$ 2,497