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Investments (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Oct. 31, 2018
Oct. 31, 2017
Oct. 31, 2018
Oct. 31, 2017
Investments, Debt and Equity Securities [Abstract]        
Proceeds of sale of marketable securities $ 3,388 $ 32,886 $ 945,685 $ 222,823