Investments (Amortized Cost And Fair Value Of Securities Available-For-Sale) (Details) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2014	Apr. 30, 2014	Jul. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost	$ 405,298	$ 424,817	$ 493,565
Gross Unrealized Gains	2,765	3,007	4,091
Available For Sale Securities, Gross Unrealized Loss, Accumulated In Investments	0	0	10,623
Investments in available-for-sale securities, Fair Value	408,063	427,824	487,033
Long-Term | Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost	401,092	420,697	489,401
Gross Unrealized Gains	2,582	2,798	3,825
Available For Sale Securities, Gross Unrealized Loss, Accumulated In Investments	0	0	10,623
Investments in available-for-sale securities, Fair Value	403,674	423,495	482,603
Long-Term | Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost	4,106	4,120	4,164
Gross Unrealized Gains	183	209	266
Available For Sale Securities, Gross Unrealized Loss, Accumulated In Investments	0	0	0
Investments in available-for-sale securities, Fair Value	4,289	4,329	4,430
Long-Term | US Treasury Bill Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost	100
Gross Unrealized Gains	0
Available For Sale Securities, Gross Unrealized Loss, Accumulated In Investments	0
Investments in available-for-sale securities, Fair Value	$ 100