Investments - Held-To-Maturity Securities (Details) - USD ($) $ in Thousands	Oct. 28, 2017	Apr. 30, 2017
Held To Maturity Securities
Amortized Cost	$ 141,679	$ 159,570
Gross Unrealized Gains	14	42
Gross Unrealized Losses	(174)	(147)
Fair Value	141,519	159,465
Municipal securities
Held To Maturity Securities
Amortized Cost	47,799	56,379
Gross Unrealized Gains	13	30
Gross Unrealized Losses	(16)	(21)
Fair Value	47,796	56,388
U.S. government securities
Held To Maturity Securities
Amortized Cost	47,630	37,055
Gross Unrealized Gains		2
Gross Unrealized Losses	(115)	(41)
Fair Value	47,515	37,016
Corporate bonds
Held To Maturity Securities
Amortized Cost	46,250	63,636
Gross Unrealized Gains	1	9
Gross Unrealized Losses	(43)	(85)
Fair Value	$ 46,208	63,560
Certificates of deposit
Held To Maturity Securities
Amortized Cost		2,500
Gross Unrealized Gains		1
Fair Value		$ 2,501