Investments - Held-To-Maturity Securities (Details) - USD ($) $ in Thousands	Jan. 27, 2018	Apr. 30, 2017
Held To Maturity Securities
Amortized Cost	$ 146,247	$ 159,570
Gross Unrealized Gains	3	42
Gross Unrealized Losses	(364)	(147)
Fair Value	145,886	159,465
Municipal securities
Held To Maturity Securities
Amortized Cost	54,986	56,379
Gross Unrealized Gains	3	30
Gross Unrealized Losses	(26)	(21)
Fair Value	54,963	56,388
U.S. government securities
Held To Maturity Securities
Amortized Cost	58,074	37,055
Gross Unrealized Gains		2
Gross Unrealized Losses	(285)	(41)
Fair Value	57,789	37,016
Corporate bonds
Held To Maturity Securities
Amortized Cost	33,187	63,636
Gross Unrealized Gains		9
Gross Unrealized Losses	(53)	(85)
Fair Value	$ 33,134	63,560
Certificates of deposit
Held To Maturity Securities
Amortized Cost		2,500
Gross Unrealized Gains		1
Fair Value		$ 2,501