Cash Equivalents and Short-Term Investments - Schedule of Contractual Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Apr. 30, 2018	Jan. 31, 2018
Amortized Cost
Amortized cost, due within one year	$ 306,045	$ 93,421
Amortized cost, due between one to five years	29,566	8,546
Amortized cost, total	335,611	101,967
Estimated Fair Value
Estimated fair value, due within one year	305,761	93,237
Estimated fair value, due between one to five years	29,524	8,528
Estimated fair value, total	$ 335,285	$ 101,765