Investments - Held-To-Maturity Securities (Details) - USD ($) $ in Thousands	Jan. 28, 2017	Apr. 30, 2016
Held To Maturity Securities
Amortized Cost	$ 162,426	$ 134,483
Gross Unrealized Gains	35	82
Gross Unrealized Losses	(169)	(39)
Fair Value	162,292	134,526
Municipal securities
Held To Maturity Securities
Amortized Cost	55,657	42,179
Gross Unrealized Gains	25	5
Gross Unrealized Losses	(43)	(7)
Fair Value	55,639	42,177
U.S. government securities
Held To Maturity Securities
Amortized Cost	35,269	28,702
Gross Unrealized Gains	5	23
Gross Unrealized Losses	(13)
Fair Value	35,261	28,725
Corporate bonds
Held To Maturity Securities
Amortized Cost	67,000	63,602
Gross Unrealized Gains	2	54
Gross Unrealized Losses	(113)	(32)
Fair Value	66,889	$ 63,624
Certificates of deposit
Held To Maturity Securities
Amortized Cost	4,500
Gross Unrealized Gains	3
Fair Value	$ 4,503