Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Mar. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 197,953	$ 164,759
Gross Unrealized Gains	9	67
Gross Unrealized Losses	(82)	(4)
Fair Market Value	197,880	164,822
US Treasury mutual fund Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	44,181	45,635
Gross Unrealized Gains	1	21
Gross Unrealized Losses	(5)
Fair Market Value	44,177	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	86,070	118,125
Gross Unrealized Gains	5	45
Gross Unrealized Losses	(29)	(4)
Fair Market Value	86,046	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	18,248	999
Gross Unrealized Gains		1
Gross Unrealized Losses	(8)
Fair Market Value	18,240	$ 1,000
Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	49,454
Gross Unrealized Gains	3
Gross Unrealized Losses	(40)
Fair Market Value	$ 49,417