Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
US Treasury mutual fund Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	$ 19,487	$ 19,487
US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	101,600	90,070
US Government-sponsored Enterprises Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value		13,996
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	121,075	123,556
Gross Unrealized Gains	14	11
Gross Unrealized Losses	(2)	(14)
Fair Market Value	121,087	123,553
Level 2 | US Treasury mutual fund Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	19,487	19,487
Fair Market Value	19,487	19,487
Level 2 | US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	101,588	90,070
Gross Unrealized Gains	14	9
Gross Unrealized Losses	(2)	(9)
Fair Market Value	$ 101,600	90,070
Level 2 | US Government-sponsored Enterprises Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		13,999
Gross Unrealized Gains		2
Gross Unrealized Losses		(5)
Fair Market Value		$ 13,996