Investments Summary of Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 2,046,735	$ 1,719,843
Available-for-sale, gross unrealized gain	2,638	15,011
Available-for-sale, gross unrealized loss	(1,997)	(474)
Available-for-sale, estimated fair value	2,047,376	1,734,380
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	866,941	741,780
Available-for-sale, gross unrealized gain	1,304	676
Available-for-sale, gross unrealized loss	(51)	(200)
Available-for-sale, estimated fair value	868,194	742,256
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	42,579	41,552
Available-for-sale, gross unrealized gain	11	155
Available-for-sale, gross unrealized loss	(516)	(9)
Available-for-sale, estimated fair value	42,074	41,698
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	9,825	9,825
Available-for-sale, gross unrealized gain	0	0
Available-for-sale, gross unrealized loss	0	0
Available-for-sale, estimated fair value	9,825	9,825
Time deposits [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		506
Available-for-sale, gross unrealized gain		0
Available-for-sale, gross unrealized loss		0
Available-for-sale, estimated fair value		506
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	1,127,390	924,818
Available-for-sale, gross unrealized gain	1,323	2,376
Available-for-sale, gross unrealized loss	(1,430)	(265)
Available-for-sale, estimated fair value	$ 1,127,283	926,929
Marketable equity securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		1,362
Available-for-sale, gross unrealized gain		11,804
Available-for-sale, gross unrealized loss		0
Available-for-sale, estimated fair value		$ 13,166

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

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-Publisher FASB

-Name Accounting Standards Codification

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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.

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