Cash Equivalents and Short-Term Investments - Schedule of Contractual Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Jul. 31, 2018	Jan. 31, 2018
Amortized Cost
Amortized cost, due within one year	$ 328,694	$ 93,421
Amortized cost, due between one to five years	14,930	8,546
Amortized cost, total	343,624	101,967
Estimated Fair Value
Estimated fair value, due within one year	328,460	93,237
Estimated fair value, due between one to five years	14,914	8,528
Estimated fair value, total	$ 343,374	$ 101,765