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Cash Equivalents and Short-Term Investments - Schedule of Amortized Costs, Unrealized Gains and Losses and Estimated Fair Value of Cash Equivalents and Short-term Investments (Details) - USD ($)
$ in Thousands
Oct. 31, 2017
Jan. 31, 2017
Cash equivalents:    
Cash equivalents, amortized cost $ 115,939 $ 10,565
Cash equivalents, estimated fair value 115,939 10,565
Investments:    
Investments, amortized cost 86,113 14,380
Investments, estimated fair value 86,043 14,390
Total short-term investments, amortized cost 86,113 14,380
Total short-term investments, unrealized gain 0 13
Total short-term investments, unrealized loss (70) (3)
Total short-term investments, estimated fair value 86,043 14,390
Amortized cost 202,052 24,945
Estimated fair value 201,982 24,955
Commercial paper    
Investments:    
Investments, amortized cost 16,973  
Investments, unrealized gain 0  
Investments, unrealized loss 0  
Investments, estimated fair value 16,973  
U.S. treasury securities    
Investments:    
Investments, amortized cost 46,570  
Investments, unrealized gain 0  
Investments, unrealized loss (57)  
Investments, estimated fair value 46,513  
Corporate debt securities    
Investments:    
Investments, amortized cost 22,570 12,842
Investments, unrealized gain 0 13
Investments, unrealized loss (13) (3)
Investments, estimated fair value 22,557 12,852
Asset-backed securities    
Investments:    
Investments, amortized cost   1,538
Investments, unrealized gain   0
Investments, unrealized loss   0
Investments, estimated fair value   $ 1,538
Money market funds    
Cash equivalents:    
Cash equivalents, amortized cost 115,939  
Cash equivalents, estimated fair value $ 115,939