Investable Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	$ 123,553us-gaap_AvailableForSaleSecurities	$ 97,424us-gaap_AvailableForSaleSecurities
US Treasury mutual fund 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	90,070	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	13,996	41,761
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	123,556	97,440
Gross Unrealized Gains	11	14
Gross Unrealized Losses	(14)	(30)
Fair Market Value	123,553	97,424
Level 2 \| US Treasury mutual fund Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	19,487	31,487
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	90,070	24,174
Gross Unrealized Gains	9	6
Gross Unrealized Losses	(9)	(4)
Fair Market Value	90,070	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	13,999	41,779
Gross Unrealized Gains	2	8
Gross Unrealized Losses	(5)	(26)
Fair Market Value	$ 13,996	$ 41,761

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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