Financial Instruments - Schedule of Gross Unrealized Losses and Fair Values for Investments that were in Continuous Unrealized Loss Position for Less Than 12 Months, Aggregated by Investments Category (Details) $ in Thousands	Jan. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less then 12 months	$ 276,192
Unrealized Loss, Less then 12 months	(282)
Fair Value Greater then 12 months	160,927
Unrealized Loss, Greater then 12 months	(956)
Fair Value Total	437,119
Unrealized Loss Total	(1,238)
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less then 12 months	11,194
Unrealized Loss, Less then 12 months	(12)
Fair Value Greater then 12 months	0
Unrealized Loss, Greater then 12 months	0
Fair Value Total	11,194
Unrealized Loss Total	(12)
Corporate Debt Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less then 12 months	83,577
Unrealized Loss, Less then 12 months	(152)
Fair Value Greater then 12 months	96,914
Unrealized Loss, Greater then 12 months	(605)
Fair Value Total	180,491
Unrealized Loss Total	(757)
US Government Agencies Debt Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less then 12 months	24,892
Unrealized Loss, Less then 12 months	(20)
Fair Value Greater then 12 months	23,600
Unrealized Loss, Greater then 12 months	(134)
Fair Value Total	48,492
Unrealized Loss Total	(154)
US Government Debt Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less then 12 months	156,529
Unrealized Loss, Less then 12 months	(98)
Fair Value Greater then 12 months	40,413
Unrealized Loss, Greater then 12 months	(217)
Fair Value Total	196,942
Unrealized Loss Total	$ (315)