Marketable Securities - Schedule of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	$ 337,614	$ 357,608
Available-for-sale securities, Gross Unrealized Gains	1,282	2,441
Available-for-sale securities, Gross Unrealized Losses	(1,579)	(75)
Available-for-sale securities, Estimated Fair Value	337,317	359,974
U.S. government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	92,577	113,033
Available-for-sale securities, Gross Unrealized Gains	130	327
Available-for-sale securities, Gross Unrealized Losses	(249)	(1)
Available-for-sale securities, Estimated Fair Value	92,458	113,359
Corporate debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	151,342	153,460
Available-for-sale securities, Gross Unrealized Gains	950	1,429
Available-for-sale securities, Gross Unrealized Losses	(648)	(30)
Available-for-sale securities, Estimated Fair Value	151,644	154,859
Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	53,212	50,852
Available-for-sale securities, Gross Unrealized Gains	185	473
Available-for-sale securities, Gross Unrealized Losses	(466)	(44)
Available-for-sale securities, Estimated Fair Value	52,931	51,281
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	39,883	39,661
Available-for-sale securities, Gross Unrealized Gains	17	211
Available-for-sale securities, Gross Unrealized Losses	(211)
Available-for-sale securities, Estimated Fair Value	39,689	39,872
Other debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	600	602
Available-for-sale securities, Gross Unrealized Gains		1
Available-for-sale securities, Gross Unrealized Losses	(5)
Available-for-sale securities, Estimated Fair Value	$ 595	$ 603

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

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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