Investments AFS (Details) (USD $)	3 Months Ended	6 Months Ended
	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2012	Mar. 31, 2012
Investments [Abstract]
Amount of short-term investments			$ 1,116,675,000	$ 823,254,000
Amount of long-term investments			154,002,000	328,586,000
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities			5,700,000
Amount of net realized gains (losses) on sales of available-for-sale marketable equity securities	(1,000,000)
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 1,700,000	$ 2,200,000