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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
Jan. 31, 2012
Apr. 30, 2011
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, Amortized Cost $ 310,780 $ 166,305
Investments in available-for-sale securities, Gross Unrealized Gains 3,694 806
Gross Unrealized Losses (287) [1] (194) [1]
Investments in available-for-sale securities, Fair Value 314,187 166,917
Short-Term [Member] | Municipal Bonds [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, Amortized Cost 2,001 3,023
Investments in available-for-sale securities, Gross Unrealized Gains 3 58
Investments in available-for-sale securities, Fair Value 2,004 3,081
Long-Term [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, Amortized Cost 308,779 163,282
Investments in available-for-sale securities, Gross Unrealized Gains 3,691 748
Gross Unrealized Losses (287) [1] (194) [1]
Investments in available-for-sale securities, Fair Value 312,183 163,836
Long-Term [Member] | Municipal Bonds [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investments in available-for-sale securities, Amortized Cost 5,260 5,312
Investments in available-for-sale securities, Gross Unrealized Gains 448 347
Investments in available-for-sale securities, Fair Value 5,708 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 303,519 157,970
Investments in available-for-sale securities, Gross Unrealized Gains 3,243 401
Gross Unrealized Losses (287) [1] (194) [1]
Investments in available-for-sale securities, Fair Value $ 306,475 $ 158,177
[1] At January 31, 2012 and April 30, 2011, we had no investments that had been in a continuous loss position for more than twelve months.