Investable Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Mar. 31, 2014
US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	$ 19,487	$ 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	58,494	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	16,473	41,761
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	94,465	97,440
Gross Unrealized Gains	20	14
Gross Unrealized Losses	(31)	(30)
Fair Market Value	94,454	97,424
Level 2 | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	19,487	31,487
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	19,487	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	58,480	24,174
Gross Unrealized Gains	17	6
Gross Unrealized Losses	(3)	(4)
Fair Market Value	58,494	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	16,498	41,779
Gross Unrealized Gains	3	8
Gross Unrealized Losses	(28)	(26)
Fair Market Value	$ 16,473	$ 41,761