Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 245,311	$ 238,198
Gross Unrealized Gains	6	6
Gross Unrealized Losses	(207)	(153)
Fair Market Value	245,110	238,051
U.S. Treasury mutual fund securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	36,138	45,199
Gross Unrealized Losses	(20)	(13)
Fair Market Value	36,118	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		1,998
Gross Unrealized Losses		(3)
Fair Market Value		1,995
Government-backed securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	113,432	90,199
Gross Unrealized Gains		1
Gross Unrealized Losses	(138)	(87)
Fair Market Value	113,294	90,113
Government-backed securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	34,970	43,484
Gross Unrealized Gains	1	5
Gross Unrealized Losses	(24)	(18)
Fair Market Value	34,947	43,471
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	58,053	55,465
Gross Unrealized Gains	1
Gross Unrealized Losses	(25)	(31)
Fair Market Value	58,029	55,434
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	2,718	1,853
Gross Unrealized Gains	4
Gross Unrealized Losses		(1)
Fair Market Value	$ 2,722	$ 1,852