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Stock-Based Compensation - Valuation Assumptions (Details)
12 Months Ended
Jan. 31, 2018
Jan. 31, 2017
Jan. 31, 2016
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life (years) 6 years 29 days 6 years 3 months 6 years 3 months
Expected volatility   52.00% 52.54%
Expected volatility, minimum 46.39%    
Expected volatility, maximum 48.77%    
Dividend yield 0.00% 0.00% 0.00%
Risk-free rate   1.66% 1.79%
Risk-free rate, minimum 1.87%    
Risk-free rate, maximum 2.70%    
Shares committed under 2017 ESPP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life (years) 11 months 0 years 0 years
Expected volatility 38.30% 0.00% 0.00%
Dividend yield 0.00% 0.00% 0.00%
Risk-free rate 1.02% 0.00% 0.00%