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Marketable Securities - Additional Information (Detail)
$ in Thousands
12 Months Ended
Jan. 31, 2016
USD ($)
Securities
Jan. 31, 2015
USD ($)
Securities
Jan. 31, 2014
USD ($)
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from sales and maturities of available-for-sale securities $ 374,057 $ 830,297 $ 451,866
Amortization of discounts and premiums, net $ 3,841 $ 6,696 10,932
Total number of securities with unrealized loss positions within the Company's portfolio | Securities 84 172  
Other (Expense) Income, Net      
Schedule of Available-for-sale Securities [Line Items]      
Net realized gain (loss) $ 43 $ 237 $ (101)