Investments and Fair Value Measurements - Securities in Unrealized Loss Position (Details) $ in Thousands	Jan. 31, 2020 USD ($)
Fair Value
Less than 12 Months	$ 136,653
12 Months or Greater	0
Total	136,653
Unrealized Losses
Less than 12 Months	(23)
12 Months or Greater	0
Total	(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	7,504
12 Months or Greater	0
Total	7,504
Unrealized Losses
Less than 12 Months	0
12 Months or Greater	0
Total	$ 0