Cash Equivalents and Marketable Securities - Summary of Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Jan. 31, 2020	Jan. 31, 2019
Amortized Cost
Due within one year	$ 273,582	$ 380,461
Due after one year through five years	135,318	46,209
Total investments in marketable securities	410,150	426,670
Estimated Fair Value
Due within one year	274,058	380,335
Due after one year through five years	136,092	46,335
Total investments in marketable securities	$ 408,900	$ 426,670