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Fair Value - Additional Information (Detail) (USD $)
 3 Months Ended 3 Months Ended
Oct. 31, 2013
Jan. 31, 2013
Oct. 31, 2012
Apr. 30, 2013
Auction Rate Securities
Jan. 31, 2013
Auction Rate Securities
Oct. 31, 2012
Auction Rate Securities
Jan. 31, 2013
Fair Value, Inputs, Level 3
Oct. 31, 2012
Fair Value, Inputs, Level 3
Oct. 31, 2013
Fair Value, Inputs, Level 3
Auction Rate Securities
Jan. 31, 2013
Fair Value, Inputs, Level 3
Auction Rate Securities
Oct. 31, 2012
Fair Value, Inputs, Level 3
Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Proceeds from sale of available-for-sale securities       $ 4,600,000         $ 4,600,000    
Amortized cost of available-for-sale securities 530,169,000 378,393,000 240,083,000             4,900,000 4,900,000
Estimated fair value of available-for-sale securities $ 530,747,000 $ 378,071,000 $ 239,832,000   $ 4,330,000 $ 4,330,000 $ 4,330,000 $ 4,330,000   $ 4,330,000 $ 4,330,000