Summary of Available for Sale (Details) (USD $)	12 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,301,989,000	$ 1,148,914,000
Available-for-sale, gross unrealized gains	6,189,000	5,048,000
Available-for-sale, gross unrealized losses	(465,000)	(2,122,000)
Available-for-sale, estimated fair value	1,307,713,000	1,151,840,000
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	558,116,000	342,025,000
Available-for-sale, gross unrealized gains	335,000	476,000
Available-for-sale, gross unrealized losses	(298,000)	(397,000)
Available-for-sale, estimated fair value	558,153,000	342,104,000
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	25,000,000	19,888,000
Available-for-sale, gross unrealized gains	146,000	234,000
Available-for-sale, gross unrealized losses	(8,000)	0
Available-for-sale, estimated fair value	25,138,000	20,122,000
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	33,459,000	10,246,000
Available-for-sale, gross unrealized gains	332,000	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	33,791,000	10,246,000
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	680,144,000	770,891,000
Available-for-sale, gross unrealized gains	5,137,000	4,150,000
Available-for-sale, gross unrealized losses	(159,000)	(937,000)
Available-for-sale, estimated fair value	685,122,000	774,104,000
Equity Securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	5,270,000	5,864,000
Available-for-sale, gross unrealized gains	239,000	188,000
Available-for-sale, gross unrealized losses	0	(788,000)
Available-for-sale, estimated fair value	5,509,000	5,264,000
Available-for-sale Securities, Gross Realized Gain (Loss)	100,000	400,000	3,200,000
Other than Temporary Impairment Losses, Investments		2,400,000
Debt Securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale Securities, Gross Realized Gain (Loss)	$ 300,000	$ 400,000

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

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+ Details

- Definition

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.

+ References+ Details

- 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.

+ References+ Details

- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

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