Investments - Held-To-Maturity Securities (Details) - USD ($) $ in Thousands	Jul. 28, 2018	Apr. 30, 2018
Held To Maturity Securities
Amortized Cost	$ 155,015	$ 152,163
Gross Unrealized Gains	9	11
Gross Unrealized Losses	(363)	(474)
Fair Value	154,661	151,700
Municipal securities
Held To Maturity Securities
Amortized Cost	16,995	37,390
Gross Unrealized Gains	8	9
Gross Unrealized Losses	(3)	(36)
Fair Value	17,000	37,363
U.S. government securities
Held To Maturity Securities
Amortized Cost	62,323	58,976
Gross Unrealized Losses	(307)	(367)
Fair Value	62,016	58,609
Corporate bonds
Held To Maturity Securities
Amortized Cost	72,697	55,797
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(53)	(71)
Fair Value	72,645	$ 55,728
Certificates of deposit
Held To Maturity Securities
Amortized Cost	3,000
Fair Value	$ 3,000