Financial Instruments Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	$ 78,662
US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	59,020	71,233
US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	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]
Available-for-sale securities, fair value disclosure	11,406
Using Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	78,662
Using Significant Other Observable Inputs (Level 2) | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	59,020	71,233
Using Significant Other Observable Inputs (Level 2) | US Government-sponsored Enterprises Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	8,236
Using Significant Other Observable Inputs (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]
Available-for-sale securities, fair value disclosure	$ 11,406