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Marketable Securities - Additional Information (Detail) (USD $)
 3 Months Ended 6 Months Ended 3 Months Ended 3 Months Ended 6 Months Ended
Jul. 31, 2013
Jul. 31, 2012
Jul. 31, 2013
Jul. 31, 2012
Jan. 31, 2013
Jul. 31, 2013
Long Term Investments
Jan. 31, 2013
Long Term Investments
Jul. 31, 2012
Long Term Investments
Apr. 30, 2013
Auction Rate Securities
Jan. 31, 2013
Auction Rate Securities
Jul. 31, 2012
Auction Rate Securities
Jan. 31, 2013
Auction Rate Securities
Long Term Investments
Jul. 31, 2012
Auction Rate Securities
Long Term Investments
Jul. 31, 2013
Other Expense Income Net
Jul. 31, 2012
Other Expense Income Net
Jul. 31, 2013
Other Expense Income Net
Jul. 31, 2012
Other Expense Income Net
Schedule of Available-for-sale Securities [Line Items]                                  
Proceeds from sale and maturities of available-for-sale securities     $ 245,759,000 $ 122,343,000                          
Net realized gain (loss)                           12,000 85,000 (202,000) 212,000
Amortization of discounts and premiums, net 2,527,000 1,243,000 4,719,000 2,478,000                          
Proceeds from sale of available-for-sale securities                 4,600,000                
Amortized cost of available-for-sale securities 442,558,000 227,773,000 442,558,000 227,773,000 378,393,000 216,833,000 150,107,000 92,058,000       4,925,000 4,925,000        
Estimated fair value of available-for-sale securities $ 442,565,000 $ 227,554,000 $ 442,565,000 $ 227,554,000 $ 378,071,000 $ 216,766,000 $ 149,585,000 $ 91,664,000   $ 4,330,000 $ 4,330,000 $ 4,330,000 $ 4,330,000