Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Mar. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 182,045	$ 164,759
Gross Unrealized Gains	215	67
Gross Unrealized Losses	(20)	(4)
Fair Market Value	182,240	164,822
US Treasury mutual fund Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		45,635
Gross Unrealized Gains		21
Fair Market Value		45,656
US Treasury mutual fund Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	37,141
Gross Unrealized Gains	9
Gross Unrealized Losses	(1)
Fair Market Value	37,149
US Treasury mutual fund Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	2,003
Gross Unrealized Gains	1
Fair Market Value	2,004
US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	90,823	118,125
Gross Unrealized Gains	39	45
Gross Unrealized Losses	(3)	(4)
Fair Market Value	90,859	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	1,999	999
Gross Unrealized Gains	1	1
Fair Market Value	2,000	$ 1,000
Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	50,079
Gross Unrealized Gains	165
Gross Unrealized Losses	(16)
Fair Market Value	$ 50,228