Investments (Details) - USD ($) $ in Thousands	Oct. 28, 2017	Apr. 30, 2017
Short-term investments:
Total held-to-maturity and short-term investments	$ 110,751	$ 119,971
Long-term investments:
Total long-term investments	33,024	42,096
Held-to-maturity securities
Short-term investments:
Total held-to-maturity and short-term investments	110,751	119,971
Long-term investments:
Total long-term investments	30,928	39,599
Held-to-maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	43,901	47,437
Long-term investments:
Total long-term investments	3,898	8,942
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	22,609	14,515
Long-term investments:
Total long-term investments	25,021	22,540
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total held-to-maturity and short-term investments	44,241	55,519
Long-term investments:
Total long-term investments	2,009	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	2,096	2,497
Available-for-sale securities. | Auction rate securities
Long-term investments:
Total long-term investments	$ 2,096	$ 2,497