Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 380,183	$ 262,173
Gross Unrealized Gains	7	6
Gross Unrealized Losses	(819)	(153)
Fair Market Value	379,371	262,026
Repurchase Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	16,750	12,000
Fair Market Value	16,750	12,000
Commercial Paper | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	36,936	42,304
Gross Unrealized Gains	2
Gross Unrealized Losses	(63)	(25)
Fair Market Value	36,875	42,279
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	5,845	11,975
Fair Market Value	5,845	11,975
U.S. Treasury mutual fund securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	18,132	45,199
Gross Unrealized Losses	(29)	(13)
Fair Market Value	18,103	45,186
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	1,998
Gross Unrealized Losses	(16)	(3)
Fair Market Value	10,937	1,995
Government-backed securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	212,255	90,199
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(538)	(87)
Fair Market Value	211,720	90,113
Government-backed securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	24,798	43,484
Gross Unrealized Gains	1	5
Gross Unrealized Losses	(12)	(18)
Fair Market Value	24,787	43,471
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	52,737	13,161
Gross Unrealized Losses	(161)	(6)
Fair Market Value	52,576	13,155
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	1,777	1,853
Gross Unrealized Gains	1
Gross Unrealized Losses		(1)
Fair Market Value	$ 1,778	$ 1,852