Fair Value Measures- Debt Securities, Available-for-sale (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 566,193	$ 558,936
Unrealized Gains	5,818	6,722
Unrealized Losses	0	147
Total	572,011	565,511
Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Total	572,011	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	469,560	415,889
Unrealized Gains	4,819	6,444
Unrealized Losses	0	0
Total	474,379	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	23,200	15,000
Unrealized Gains	29	20
Unrealized Losses	0	34
Total	23,229	14,986
Commercial paper | Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,173	13,318
Unrealized Gains	110	16
Unrealized Losses	0	0
Total	17,283	13,334
Corporate notes and bonds | Short-term investments:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	56,260	60,211
Unrealized Gains	860	221
Unrealized Losses	0	113
Total	$ 57,120	$ 60,319