Components of Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Sep. 30, 2012	Mar. 31, 2012
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 75,187	$ 96,423
Unrealized Gains (Losses)	32	(162)
Fair Value	75,219	96,261
U.S. Government And Municipal Obligations
Cash, Cash Equivalents and Marketable Securities [Line Items]
Fair Value	29,817	31,635
Commercial Paper
Cash, Cash Equivalents and Marketable Securities [Line Items]
Fair Value	26,671	22,469
Certificates of Deposit
Cash, Cash Equivalents and Marketable Securities [Line Items]
Fair Value	5,522	3,207
Short-term marketable securities
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	69,576	79,599
Unrealized Gains (Losses)	28	18
Fair Value	69,604	79,617
Short-term marketable securities \| U.S. Government And Municipal Obligations
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	27,190	17,779
Unrealized Gains (Losses)	16	20
Fair Value	27,206	17,799
Short-term marketable securities \| Commercial Paper
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	26,668	22,469
Unrealized Gains (Losses)	3	0
Fair Value	26,671	22,469
Short-term marketable securities \| Corporate Bonds
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	10,197	18,531
Unrealized Gains (Losses)	8	(1)
Fair Value	10,205	18,530
Short-term marketable securities \| Certificates of Deposit
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	5,521	3,208
Unrealized Gains (Losses)	1	(1)
Fair Value	5,522	3,207
Short-term marketable securities \| Auction Rate Securities
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost		17,612
Unrealized Gains (Losses)		0
Fair Value		17,612
Long-term marketable securities
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	5,611	16,824
Unrealized Gains (Losses)	4	(180)
Fair Value	5,615	16,644
Long-term marketable securities \| U.S. Government And Municipal Obligations
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	2,607	13,828
Unrealized Gains (Losses)	4	8
Fair Value	2,611	13,836
Long-term marketable securities \| Corporate Bonds
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	3,004	1,345
Unrealized Gains (Losses)	0	2
Fair Value	3,004	1,347
Long-term marketable securities \| Auction Rate Securities
Cash, Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost		1,651
Unrealized Gains (Losses)		(190)
Fair Value		$ 1,461

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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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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- Definition

The aggregate net change in the difference between the fair value and the carrying value, or in the comparative fair values, of marketable securities categorized as trading held at each balance sheet date, that was included in earnings for the period, which may have arisen from (a) securities classified as trading, (b) the unrealized holding gain (loss) on held-to-maturity securities transferred to the trading security category, and (c) the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) on available-for-sale securities transferred to trading securities during the period.

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