Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 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	95,002	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	1,501	13,996
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	115,983	123,556
Gross Unrealized Gains	11	11
Gross Unrealized Losses	(4)	(14)
Fair Market Value	115,990	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	94,996	90,070
Gross Unrealized Gains	10	9
Gross Unrealized Losses	(4)	(9)
Fair Market Value	95,002	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	1,500	13,999
Gross Unrealized Gains	1	2
Gross Unrealized Losses		(5)
Fair Market Value	$ 1,501	$ 13,996