Investable Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Mar. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	$ 86,761	$ 78,662
US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	36,986	59,020
US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	23,964	8,236
US Government-sponsored Enterprises Debt Securities | Other Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	25,811	11,406
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	86,749	78,660
Gross Unrealized Gains	14	4
Gross Unrealized Losses	(2)	(2)
Fair Market Value	86,761	78,662
Level 2 | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	36,986	59,020
Fair Market Value	36,986	59,020
Level 2 | US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	23,957	8,235
Gross Unrealized Gains	7	1
Fair Market Value	23,964	8,236
Level 2 | US Government-sponsored Enterprises Debt Securities | Other Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	25,806	11,405
Gross Unrealized Gains	7	3
Gross Unrealized Losses	(2)	(2)
Fair Market Value	$ 25,811	$ 11,406