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Income Taxes (Reconciliation of Income Taxes Computed at Federal Statutory Income Tax Rate and Provision for (Benefit from) Income Taxes) (Detail)
12 Months Ended
Jan. 31, 2019
Jan. 31, 2018
Jan. 31, 2017
Income Tax Disclosure [Abstract]      
Federal statutory rate 21.00% 33.80% 35.00%
Effect of:      
Foreign income at other than U.S. rates (8.90%) (26.50%) (18.50%)
Intercompany transactions 3.70% 10.20% 4.20%
Research tax credits 12.60% 9.10% 6.40%
State taxes, net of federal benefit (0.10%) 0.00% 0.00%
U.S. corporate tax rate reduction 0.00% (81.30%) 0.00%
Changes in valuation allowance (39.70%) 33.00% (20.40%)
Stock compensation 12.70% 20.00% (6.10%)
Other 0.00% (0.40%) (0.40%)
Total 1.30% (2.10%) 0.20%