Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Mar. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 164,759	$ 123,556
Gross Unrealized Gains	67	11
Gross Unrealized Losses	(4)	(14)
Fair Market Value	164,822	123,553
US Treasury mutual fund Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	45,635	19,487
Gross Unrealized Gains	21
Fair Market Value	45,656	19,487
US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	118,125	90,070
Gross Unrealized Gains	45	9
Gross Unrealized Losses	(4)	(9)
Fair Market Value	118,166	90,070
US Government-sponsored Enterprises Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	999	13,999
Gross Unrealized Gains	1	2
Gross Unrealized Losses		(5)
Fair Market Value	$ 1,000	$ 13,996