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Stockholders' Equity - Valuation Assumptions (Details) - $ / shares
12 Months Ended
Jan. 31, 2020
Jan. 31, 2019
Jan. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate, minimum 160.00% 200.00% 110.00%
Risk-free interest rate, maximum 210.00% 260.00% 170.00%
Weighted-average fair value per share of grants (in dollars per share) $ 0.001    
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility, minimum 2700.00% 2700.00% 2800.00%
Volatility, maximum 3000.00% 2800.00% 3100.00%
Estimated life 3 years 6 months 3 years 6 months 3 years 6 months
Risk-free interest rate, minimum 160.00% 250.00% 140.00%
Risk-free interest rate, maximum 250.00% 300.00% 230.00%
Weighted-average fair value per share of grants (in dollars per share) $ 39.59 $ 28.89 $ 22.71
ESPP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility, minimum 2800.00% 2300.00% 2100.00%
Volatility, maximum 3300.00% 2600.00% 2800.00%
Estimated life 9 months 9 months 9 months
Weighted-average fair value per share of grants (in dollars per share) $ 41.43 $ 32.90 $ 23.64