Fair Value Measurements - Schedule of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Apr. 30, 2019	Jan. 31, 2019
Financial Assets:
Marketable securities	$ 107,389	$ 112,777
Treasury Bills
Financial Assets:
Marketable securities		1,699
Fair Value ,Recurring Basis
Financial Assets:
Cash equivalents	456,401	78
Marketable securities	107,389	112,777
Prepaid expenses and other current assets	200	200
Other assets, non-current	2,144	2,144
Total financial assets	566,134	115,199
Financial Liabilities:
Convertible promissory notes – derivative liabilities		163
Total financial liabilities		163
Fair Value ,Recurring Basis | Convertible Promissory Notes
Financial Liabilities:
Convertible promissory notes – derivative liabilities		163
Fair Value ,Recurring Basis | Treasury Bills
Financial Assets:
Cash equivalents	253,796
Marketable securities		1,699
Fair Value ,Recurring Basis | Corporate Bond
Financial Assets:
Marketable securities	53,739	53,214
Fair Value ,Recurring Basis | Agency Bonds
Financial Assets:
Marketable securities	21,458	32,604
Fair Value ,Recurring Basis | Certificates Of Deposit
Financial Assets:
Prepaid expenses and other current assets	200	200
Other assets, non-current	2,144	2,144
Fair Value ,Recurring Basis | Money Market Funds
Financial Assets:
Cash equivalents	202,605	78
Fair Value ,Recurring Basis | Commercial Paper
Financial Assets:
Marketable securities		1,243
Fair Value ,Recurring Basis | U.S. Government Agency Securities
Financial Assets:
Marketable securities	32,192	24,017
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 1
Financial Assets:
Cash equivalents	202,605	78
Marketable securities	0
Prepaid expenses and other current assets	0
Other assets, non-current	0
Total financial assets	202,605	78
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 1 | Treasury Bills
Financial Assets:
Cash equivalents	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 1 | Corporate Bond
Financial Assets:
Marketable securities	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 1 | Agency Bonds
Financial Assets:
Marketable securities	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 1 | Certificates Of Deposit
Financial Assets:
Prepaid expenses and other current assets	0
Other assets, non-current	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 1 | Money Market Funds
Financial Assets:
Cash equivalents	202,605	78
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 1 | U.S. Government Agency Securities
Financial Assets:
Marketable securities	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2
Financial Assets:
Cash equivalents	253,796
Marketable securities	107,389	112,777
Prepaid expenses and other current assets	200	200
Other assets, non-current	2,144	2,144
Total financial assets	363,529	115,121
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2 | Treasury Bills
Financial Assets:
Cash equivalents	253,796
Marketable securities		1,699
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2 | Corporate Bond
Financial Assets:
Marketable securities	53,739	53,214
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2 | Agency Bonds
Financial Assets:
Marketable securities	21,458	32,604
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2 | Certificates Of Deposit
Financial Assets:
Prepaid expenses and other current assets	200	200
Other assets, non-current	2,144	2,144
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2 | Money Market Funds
Financial Assets:
Cash equivalents	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2 | Commercial Paper
Financial Assets:
Marketable securities		1,243
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 2 | U.S. Government Agency Securities
Financial Assets:
Marketable securities	32,192	24,017
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3
Financial Assets:
Cash equivalents	0
Marketable securities	0
Prepaid expenses and other current assets	0
Other assets, non-current	0
Total financial assets	0
Financial Liabilities:
Convertible promissory notes – derivative liabilities		163
Total financial liabilities		163
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3 | Convertible Promissory Notes
Financial Liabilities:
Convertible promissory notes – derivative liabilities		$ 163
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3 | Treasury Bills
Financial Assets:
Cash equivalents	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3 | Corporate Bond
Financial Assets:
Marketable securities	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3 | Agency Bonds
Financial Assets:
Marketable securities	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3 | Certificates Of Deposit
Financial Assets:
Prepaid expenses and other current assets	0
Other assets, non-current	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3 | Money Market Funds
Financial Assets:
Cash equivalents	0
Fair Value ,Recurring Basis | Fair Value, Inputs, Level 3 | U.S. Government Agency Securities
Financial Assets:
Marketable securities	$ 0