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	Jan. 31, 2018	Jan. 31, 2017
Cash equivalents:
Cash equivalents, amortized cost	$ 90,770	$ 10,565
Cash equivalents, estimated fair value	90,770	10,565
Short-term investments:
Investments, amortized cost	101,967	14,380
Investments, estimated fair value	101,765	14,390
Total short-term investments, amortized cost	101,967	14,380
Total short-term investments, unrealized gain	0	13
Total short-term investments, unrealized loss	(202)	(3)
Total short-term investments, estimated fair value	101,765	14,390
Amortized cost	192,737	24,945
Estimated fair value	192,535	24,955
Commercial paper
Short-term investments:
Investments, amortized cost	15,946
Investments, unrealized gain	0
Investments, unrealized loss	0
Investments, estimated fair value	15,946
U.S. treasury securities
Short-term investments:
Investments, amortized cost	61,896
Investments, unrealized gain	0
Investments, unrealized loss	(158)
Investments, estimated fair value	61,738
Asset-backed securities
Short-term investments:
Investments, amortized cost		1,538
Investments, unrealized gain		0
Investments, unrealized loss		0
Investments, estimated fair value		1,538
Corporate debt securities
Short-term investments:
Investments, amortized cost	24,125	12,842
Investments, unrealized gain	0	13
Investments, unrealized loss	(44)	(3)
Investments, estimated fair value	24,081	$ 12,852
Money market funds
Cash equivalents:
Cash equivalents, amortized cost	90,770
Cash equivalents, estimated fair value	$ 90,770