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Marketable Securities - Additional Information (Detail) (USD $)
 3 Months Ended 9 Months Ended 3 Months Ended 3 Months Ended 9 Months Ended
Oct. 31, 2013
Oct. 31, 2012
Oct. 31, 2013
Oct. 31, 2012
Jan. 31, 2013
Oct. 31, 2013
Long Term Investments
Jan. 31, 2013
Long Term Investments
Oct. 31, 2012
Long Term Investments
Apr. 30, 2013
Auction Rate Securities
Jan. 31, 2013
Auction Rate Securities
Oct. 31, 2012
Auction Rate Securities
Jan. 31, 2013
Auction Rate Securities
Long Term Investments
Oct. 31, 2012
Auction Rate Securities
Long Term Investments
Oct. 31, 2013
Other Expense Income Net
Oct. 31, 2012
Other Expense Income Net
Oct. 31, 2013
Other Expense Income Net
Oct. 31, 2012
Other Expense Income Net
Schedule of Available-for-sale Securities [Line Items]                                  
Proceeds from sale and maturities of available-for-sale securities     $ 326,103,000 $ 175,991,000                          
Net realized gain (loss)                           37,000 36,000 (165,000) 249,000
Amortization of discounts and premiums, net 2,992,000 1,179,000 7,711,000 3,657,000                          
Proceeds from sale of available-for-sale securities                 4,600,000                
Amortized cost of available-for-sale securities 530,169,000 240,083,000 530,169,000 240,083,000 378,393,000 302,730,000 150,107,000 94,235,000       4,925,000 4,925,000        
Estimated fair value of available-for-sale securities $ 530,747,000 $ 239,832,000 $ 530,747,000 $ 239,832,000 $ 378,071,000 $ 303,200,000 $ 149,585,000 $ 93,767,000   $ 4,330,000 $ 4,330,000 $ 4,330,000 $ 4,330,000