Investments - Held-To-Maturity Securities (Details) - USD ($) $ in Thousands	Jul. 29, 2017	Apr. 30, 2017
Held To Maturity Securities
Amortized Cost	$ 141,622	$ 159,570
Gross Unrealized Gains	39	42
Gross Unrealized Losses	(120)	(147)
Fair Value	141,541	159,465
Municipal securities
Held To Maturity Securities
Amortized Cost	41,915	56,379
Gross Unrealized Gains	32	30
Gross Unrealized Losses	(6)	(21)
Fair Value	41,941	56,388
U.S. government securities
Held To Maturity Securities
Amortized Cost	45,144	37,055
Gross Unrealized Gains	2	2
Gross Unrealized Losses	(48)	(41)
Fair Value	45,098	37,016
Corporate bonds
Held To Maturity Securities
Amortized Cost	52,063	63,636
Gross Unrealized Gains	4	9
Gross Unrealized Losses	(66)	(85)
Fair Value	52,001	63,560
Certificates of deposit
Held To Maturity Securities
Amortized Cost	2,500	2,500
Gross Unrealized Gains	1	1
Fair Value	$ 2,501	$ 2,501