Investments and Fair Value Measurements - Securities in Unrealized Loss Position (Details) - USD ($) $ in Thousands	Apr. 30, 2020	Jan. 31, 2020
Fair Value
Less than 12 Months	$ 0	$ 136,653
12 Months or Greater	3,500	0
Total	3,500	136,653
Unrealized Losses
Less than 12 Months	0	(23)
12 Months or Greater	(1)	0
Total	(1)	(23)
US Treasury Securities
Fair Value
Less than 12 Months		129,149
12 Months or Greater		0
Total		129,149
Unrealized Losses
Less than 12 Months		(23)
12 Months or Greater		0
Total		(23)
Corporate Bonds
Fair Value
Less than 12 Months	0	7,504
12 Months or Greater	3,500	0
Total	3,500	7,504
Unrealized Losses
Less than 12 Months	0	0
12 Months or Greater	(1)	0
Total	$ (1)	$ 0