Summary of Available for Sale (Details) (USD $)											3 Months Ended		3 Months Ended	6 Months Ended
Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2013 Government agency bonds [Member]	Mar. 31, 2013 Government agency bonds [Member]	Sep. 30, 2013 Municipal bonds [Member]	Mar. 31, 2013 Municipal bonds [Member]	Sep. 30, 2013 Auction rate securities [Member]	Mar. 31, 2013 Auction rate securities [Member]	Sep. 30, 2013 Corporate bonds and debt [Member]	Mar. 31, 2013 Corporate bonds and debt [Member]	Sep. 30, 2013 Equity Securities [Member]	Mar. 31, 2013 Equity Securities [Member]	Sep. 30, 2013 Debt Securities [Member]	Sep. 30, 2013 Debt Securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,641,380,000	$ 1,301,989,000	$ 781,230,000	$ 558,116,000	$ 54,286,000	$ 25,000,000	$ 9,829,000	$ 33,459,000	$ 796,035,000	$ 680,144,000		$ 5,270,000
Available-for-sale, gross unrealized gains	3,231,000	6,189,000	269,000	335,000	60,000	146,000	0	332,000	2,902,000	5,137,000		239,000
Available-for-sale, gross unrealized losses	(7,246,000)	(465,000)	(5,467,000)	(298,000)	(575,000)	(8,000)	0	0	(1,204,000)	(159,000)		0
Available-for-sale, estimated fair value	1,637,365,000	1,307,713,000	776,032,000	558,153,000	53,771,000	25,138,000	9,829,000	33,791,000	797,733,000	685,122,000		5,509,000
Realized gain (loss) on sale of available-for-sale securities											$ 1,000,000		$ 1,100,000	$ 1,200,000

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

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

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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