Investments (Amortized Cost And Fair Value Of Securities Available-For-Sale) (Details) - USD ($) $ in Thousands	Jul. 31, 2015	Apr. 30, 2015	Jul. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 395,410	$ 427,688	$ 405,298
Gross Unrealized Gains	12,016	14,045	2,765
Gross Unrealized Losses	0	(24)	0
Available-for-sale Securities, Current	406,360	439,625	403,774
Fair Value	407,426	441,709	408,063
Long-Term | Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	392,285	421,035	401,092
Gross Unrealized Gains	11,936	13,889	2,582
Gross Unrealized Losses	0	0	0
Available-for-sale Securities, Current	404,221
Fair Value		434,924	403,674
Long-Term | Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,025	4,062	4,106
Gross Unrealized Gains	80	109	183
Gross Unrealized Losses	0	(24)	0
Fair Value	3,105	4,147	4,289
Long-Term | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,491
Gross Unrealized Gains		47
Gross Unrealized Losses		0
Fair Value		2,538
Long-Term | US Treasury Bill Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	100	100	100
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Fair Value	$ 100	$ 100	$ 100