Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 274,808	$ 238,198
Gross Unrealized Gains	2	6
Gross Unrealized Losses	(195)	(153)
Fair Market Value	274,615	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	36,178	45,199
Gross Unrealized Losses	(9)	(13)
Fair Market Value	36,169	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	124,654	90,199
Gross Unrealized Gains		1
Gross Unrealized Losses	(87)	(87)
Fair Market Value	124,567	90,113
Government-backed securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	48,186	43,484
Gross Unrealized Gains		5
Gross Unrealized Losses	(61)	(18)
Fair Market Value	48,125	43,471
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	57,955	55,465
Gross Unrealized Gains	2
Gross Unrealized Losses	(32)	(31)
Fair Market Value	57,925	55,434
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	7,835	1,853
Gross Unrealized Losses	(6)	(1)
Fair Market Value	$ 7,829	$ 1,852