Investments - Held-To-Maturity Securities (Details) - USD ($) $ in Thousands	Apr. 30, 2018	Apr. 30, 2017
Held To Maturity Securities
Amortized Cost	$ 152,163	$ 159,570
Gross Unrealized Gains	11	42
Gross Unrealized Losses	(474)	(147)
Fair Value	151,700	159,465
Municipal securities
Held To Maturity Securities
Amortized Cost	37,390	56,379
Gross Unrealized Gains	9	30
Gross Unrealized Losses	(36)	(21)
Fair Value	37,363	56,388
U.S. government securities
Held To Maturity Securities
Amortized Cost	58,976	37,055
Gross Unrealized Gains		2
Gross Unrealized Losses	(367)	(41)
Fair Value	58,609	37,016
Corporate bonds
Held To Maturity Securities
Amortized Cost	55,797	63,636
Gross Unrealized Gains	2	9
Gross Unrealized Losses	(71)	(85)
Fair Value	$ 55,728	63,560
Certificates of deposit
Held To Maturity Securities
Amortized Cost		2,500
Gross Unrealized Gains		1
Fair Value		$ 2,501