Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 300,830	$ 238,198
Gross Unrealized Gains	3	6
Gross Unrealized Losses	(597)	(153)
Fair Market Value	300,236	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	21,068	45,199
Gross Unrealized Losses	(25)	(13)
Fair Market Value	21,043	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	152,586	90,199
Gross Unrealized Gains		1
Gross Unrealized Losses	(343)	(87)
Fair Market Value	152,243	90,113
Government-backed securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	47,587	43,484
Gross Unrealized Gains		5
Gross Unrealized Losses	(96)	(18)
Fair Market Value	47,491	43,471
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	77,592	55,465
Gross Unrealized Gains	3
Gross Unrealized Losses	(130)	(31)
Fair Market Value	77,465	55,434
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	1,997	1,853
Gross Unrealized Losses	(3)	(1)
Fair Market Value	$ 1,994	$ 1,852