Investments (Details) - USD ($) $ in Thousands	Jul. 29, 2017	Apr. 30, 2017
Short-term investments:
Total held-to-maturity and short-term investments	$ 107,831	$ 119,971
Long-term investments:
Total long-term investments	35,844	42,096
Held-to-maturity securities
Short-term investments:
Total held-to-maturity and short-term investments	107,831	119,971
Long-term investments:
Total long-term investments	33,791	39,599
Held-to-maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	37,164	47,437
Long-term investments:
Total long-term investments	4,751	8,942
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	16,104	14,515
Long-term investments:
Total long-term investments	29,040	22,540
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total held-to-maturity and short-term investments	52,063	55,519
Long-term investments:
Total long-term investments		8,117
Held-to-maturity securities | Certificates of deposit
Short-term investments:
Total held-to-maturity and short-term investments	2,500	2,500
Available-for-sale securities
Long-term investments:
Total long-term investments	2,053	2,497
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments	$ 2,053	$ 2,497