Fair Value Measures- Debt Securities, Available-for-sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 558,833	$ 558,936
Unrealized Gains	4,147	6,722
Unrealized Losses	(41)	(147)
Total	562,939	565,511
Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Total	562,939	512,574
U.S. treasury securities | Cash and cash equivalents:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		27,923
Unrealized Gains		15
Unrealized Losses		0
Total		27,938
U.S. treasury securities | Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	460,188	415,889
Unrealized Gains	3,410	6,444
Unrealized Losses	(41)	0
Total	463,557	422,333
U.S. government agencies | Cash and cash equivalents:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		24,995
Unrealized Gains		4
Unrealized Losses		0
Total		24,999
U.S. government agencies | Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,600
Unrealized Gains		2
Unrealized Losses		0
Total		1,602
Certificates of deposit | Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26,700	15,000
Unrealized Gains	13	20
Unrealized Losses	0	(34)
Total	26,713	14,986
Commercial paper | Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,741	13,318
Unrealized Gains	47	16
Unrealized Losses	0	0
Total	20,788	13,334
Corporate notes and bonds | Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51,204	60,211
Unrealized Gains	677	221
Unrealized Losses	0	(113)
Total	$ 51,881	$ 60,319