Investments AFS (Details) (USD $)	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
Investments [Abstract]
Amount of short-term investments	$ 823,254,000	$ 539,572,000
Amount of long-term investments	328,586,000	464,838,000
Impairment amount in value of auction rate securities failed		1,600,000
Net recognized gain on auction rate securities	300,000
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities	4,600,000	3,500,000
Amount of net realized gains (losses) on sales of available-for-sale marketable equity securities	400,000	3,200,000
Amount of net realized gains (losses) on sales of available-for-sale debt securities	400,000		1,200,000
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 2,400,000