Investable Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 238,198	$ 164,759
Gross Unrealized Gains	6	67
Gross Unrealized Losses	(153)	(4)
Fair Market Value	238,051	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	45,199
Gross Unrealized Losses	(13)
Fair Market Value	45,186
US Treasury mutual fund Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	1,998
Gross Unrealized Losses	(3)
Fair Market Value	1,995
US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	90,199	118,125
Gross Unrealized Gains	1	45
Gross Unrealized Losses	(87)	(4)
Fair Market Value	90,113	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	43,484	999
Gross Unrealized Gains	5	1
Gross Unrealized Losses	(18)
Fair Market Value	43,471	$ 1,000
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	55,465
Gross Unrealized Losses	(31)
Fair Market Value	55,434
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	1,853
Gross Unrealized Losses	(1)
Fair Market Value	$ 1,852