Gross Unrealized Losses and Fair Values (Details) $ in Thousands	Apr. 30, 2019 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	$ 81,353
Unrealized Loss, Less than 12 Months	(24)
Fair Value, Greater than 12 months	120,754
Unrealized Loss, Greater than 12 months	(491)
Fair Value, Total	202,107
Unrealized Loss, Total	(515)
U.S. government treasury notes
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	33,192
Unrealized Loss, Less than 12 Months	(6)
Fair Value, Greater than 12 months	31,518
Unrealized Loss, Greater than 12 months	(119)
Fair Value, Total	64,710
Unrealized Loss, Total	(125)
U.S. government agencies
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	16,083
Unrealized Loss, Less than 12 Months	(2)
Fair Value, Greater than 12 months	22,928
Unrealized Loss, Greater than 12 months	(77)
Fair Value, Total	39,011
Unrealized Loss, Total	(79)
Corporate debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	31,210
Unrealized Loss, Less than 12 Months	(16)
Fair Value, Greater than 12 months	66,308
Unrealized Loss, Greater than 12 months	(295)
Fair Value, Total	97,518
Unrealized Loss, Total	(311)
Asset-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value, Less than 12 months	868
Unrealized Loss, Less than 12 Months	0
Fair Value, Greater than 12 months	0
Unrealized Loss, Greater than 12 months	0
Fair Value, Total	868
Unrealized Loss, Total	$ 0