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Investments (Detail) - USD ($)
3 Months Ended
Apr. 30, 2019
Apr. 30, 2018
Investments, Debt and Equity Securities [Abstract]    
Proceeds of sale of marketable securities, debt $ 0 $ 27,000,000
Proceeds of sale of marketable securities, equity 51,000,000 $ 0
Net realized gains (losses) recognized on equity investments sold $ 7,000,000