Investments (Details) - USD ($) $ in Thousands	Jan. 28, 2017	Apr. 30, 2016
Short-term investments:
Total held-to-maturity and short-term investments	$ 121,095	$ 103,404
Long-term investments:
Total long-term investments	43,749	33,859
Held-to-maturity securities
Short-term investments:
Total held-to-maturity and short-term investments	121,095	103,404
Long-term investments:
Total long-term investments	41,331	31,079
Held-to-maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	40,211	42,179
Long-term investments:
Total long-term investments	15,446
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	19,720	21,184
Long-term investments:
Total long-term investments	15,549	7,518
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total held-to-maturity and short-term investments	56,664	40,041
Long-term investments:
Total long-term investments	10,336	23,561
Held-to-maturity securities | Certificates of deposit
Short-term investments:
Total held-to-maturity and short-term investments	4,500
Available-for-sale securities
Long-term investments:
Total long-term investments	2,418	2,780
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments	$ 2,418	$ 2,780