Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 416,372	$ 380,183
Gross Unrealized Gains	60	7
Gross Unrealized Losses	(311)	(819)
Fair Market Value	416,121	379,371
Repurchase Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	6,000	16,750
Fair Market Value	6,000	16,750
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	36,146	5,845
Fair Market Value	36,146	5,845
Commercial Paper | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	32,792	36,936
Gross Unrealized Gains		2
Gross Unrealized Losses	(52)	(63)
Fair Market Value	32,740	36,875
U.S. Treasury mutual fund securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	43,710	18,132
Gross Unrealized Gains	1
Gross Unrealized Losses	(37)	(29)
Fair Market Value	43,674	18,103
U.S. Treasury mutual fund securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		10,953
Gross Unrealized Losses		(16)
Fair Market Value		10,937
Government-backed securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	183,531	212,255
Gross Unrealized Gains	13	3
Gross Unrealized Losses	(152)	(538)
Fair Market Value	183,392	211,720
Government-backed securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		24,798
Gross Unrealized Gains		1
Gross Unrealized Losses		(12)
Fair Market Value		24,787
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	114,193	52,737
Gross Unrealized Gains	46
Gross Unrealized Losses	(70)	(161)
Fair Market Value	$ 114,169	52,576
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		1,777
Gross Unrealized Gains		1
Fair Market Value		$ 1,778