Investable Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	$ 86,218	$ 97,424
US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	19,486	31,487
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	32,972	24,176
US Government-sponsored Enterprises Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	33,760	41,761
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	86,207	97,440
Gross Unrealized Gains	22	14
Gross Unrealized Losses	(11)	(30)
Fair Market Value	86,218	97,424
Level 2 | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	19,486	31,487
Fair Market Value	19,486	31,487
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	32,963	24,174
Gross Unrealized Gains	11	6
Gross Unrealized Losses	(2)	(4)
Fair Market Value	32,972	24,176
Level 2 | US Government-sponsored Enterprises Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	33,758	41,779
Gross Unrealized Gains	11	8
Gross Unrealized Losses	(9)	(26)
Fair Market Value	$ 33,760	$ 41,761