Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 184,686	$ 164,759
Gross Unrealized Gains	215	67
Gross Unrealized Losses	(1)	(4)
Fair Market Value	184,900	164,822
US Treasury mutual fund Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	45,595	45,635
Gross Unrealized Gains	23	21
Fair Market Value	45,618	45,656
US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	112,642	118,125
Gross Unrealized Gains	73	45
Gross Unrealized Losses	(1)	(4)
Fair Market Value	112,714	118,166
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
Gross Unrealized Gains		1
Fair Market Value		$ 1,000
Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	26,449
Gross Unrealized Gains	119
Fair Market Value	$ 26,568