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Investments In Available-For-Sale Securities (Amortized Cost And Fair Value Of Securities Available-For-Sale) (Details) (USD $)
In Thousands, unless otherwise specified
Oct. 31, 2011
Apr. 30, 2011
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, amortized cost $ 304,442 $ 166,305
Investments in available-for-sale securities, gross unrealized gains 4,106 806
Investments in available-for-sale securities, gross unrealized losses (209) [1] (194) [1]
Investments in available-for-sale securities, fair value 308,339 166,917
Short-Term [Member] | Municipal Bonds [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, amortized cost 2,008 3,023
Investments in available-for-sale securities, gross unrealized gains 21 58
Investments in available-for-sale securities, fair value 2,029 3,081
Long-Term [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, amortized cost 302,434 163,282
Investments in available-for-sale securities, gross unrealized gains 4,085 748
Investments in available-for-sale securities, gross unrealized losses (209) [1] (194) [1]
Investments in available-for-sale securities, fair value 306,310 163,836
Long-Term [Member] | Municipal Bonds [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, amortized cost 5,277 5,312
Investments in available-for-sale securities, gross unrealized gains 369 347
Investments in available-for-sale securities, fair value 5,646 5,659
Long-Term [Member] | Mortgage-Backed Securities [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, amortized cost 297,157 157,970
Investments in available-for-sale securities, gross unrealized gains 3,716 401
Investments in available-for-sale securities, gross unrealized losses (209) [1] (194) [1]
Investments in available-for-sale securities, fair value $ 300,664 $ 158,177
[1] At October 31, 2011 and April 30, 2011, we had no investments that had been in a continuous loss position for more than twelve months.